Trying to resolve the mysteries surrounding the origins of the author known as Wyndham Martyn is still ongoing and now has some of the finest minds in literary detection working on it. John Herrington sent me some additional material from which I have made an attempt to work out some kind of timeline for Martyn's career. There are two main sources of information, both located I believe by Victor Berch.
First up, Martyn's entry in the volume Who's Who Among North American Authors:
Martyn, William (pen name William Grenvil); mining engineer; b. London, Eng. July 1875; son of Richard Wyndham and Clarice (Camplyn) Martyn; educ. Oxford, London, and Brusselles School of Mines and London College of Assaying. Degrees: M.A., M.E.; married Amy Elizabeth Stanley, June 1901. Contributor to Cosmopolitan, Red Book, Hearst's International, Liberty, Munsey's, Everybody's. Editor of New Yorker (1906).
I'm reasonably sure that some of this information is false. For instance, there's no record of an Amy Elizabeth Stanley being married in the UK in June 1901, but there is for the second (June) quarter of 1900 (ref. Marriages: Jun 1900, Strand 1b 1154) -- to William Henry M. Hosken. As we will discover shortly, the Hosken's moved to America in around 1904/05, by which time they had a young daughter Phylis -- almost certainly Phyllis Barbara Hosken born in 1903 (ref. Births: Dec 1903, Kingston 2a 465).
According to an interview that appeared in the Boston Daily Globe Martyn was, at the time (1912), "making his home in and around Boston." Please note that the copy I have seen was a PDF of a scan of a microfilm copy and it is a little difficult to read in places. Uncertain words are indicated [thus], whilst my own comments appear [[thus]]. In the interview, Martyn reveals that...
"As an English lad of 5 I wrote a hymn which has fortunately not survived. When I was 11 and at a preparatory school getting ready for examination for the Royal Navy (which I never entered owing to illness), I lampooned an older boy in verse. Lacking proper appreciation of my juvenilesque satire, he knocked a tooth out. I fled early for the profession of letters.
"A dozen years later I was staying in Germany and had many instances of the absurdity of militarism brought to my notice. I wrote an article in a London review about it and received 17 challenges to duels on that account..."
[[If we have our timeline correct, this would have been at around the age of 23 and the article would have appeared around 1897/98.]]
"It was a few years later when one fine morning I found that a defaulting lawyer and a bank that failed had brought my finances to a parlous state. I tossed up to see whether I should go to Australia or New York. America was heads, and won. From New York I went westward. Here I fell in with a professional partner who said he had made his pile in horses and wanted to retire. he gave me chloral hydrate in abominable whiskey and retired as he desired to do. When I could not ride the top of cars I walked back to New York.
"It took a long while and I engaged in new professions en route. I punched cows for $35 a month for one thing and talked yegg philosophy with fellow voyagers. There was one man, now dead -- he was shot robbing a post-office -- who wanted me to aid him in confidence games in Chicago.
"I was to enact the star role, and he was most grieved when I declined...
[[In our timeline, this would appear to have been around 1904/05.]]
"When, finally, I was back in New York, I brought of letters of introduction given me by a delightful American-Consul in Europe to some New York editors. They did not, nowever, procure me a high-salaried position, but they turned my mind to literary pursuits again and I began to write. I think doggerel verses brought me my first money. Then the editor and publisher of a popular magazine, who has been a very good friend to me, bought stories. I also sold big game stories while the [taste?] and finally after much tribulation found my feet.
[[The earliest traced of Martyn's stories began appearing in Pearson's Magazine in April 1906 under the name W. H. G. Wyndham Martyn. This was the American edition of the famous British magazine but also included new material. The publisher was J. J. Little of New York and the editor thought to be Arthur W. Little.]]
"I have been from time to time connected with newspapers, but it is toward fiction that I have [again?] turned, and like all novelists I feel that the drama needs me.
[[Victor Berch has traced an unpublished play registered in the Catalog of Copyrights: The Upbringing of Eleanor in 3 acts, copyrighted August 3, 1910. At the time, Martyn gave his address as Edgewater, New Jersey.]]
"My publishers have announced 'All the World to Nothing' is my [first] novel. They did it in good faith. [But it] really is my third. The first is a blood-curdling story of political intrigue in the Balkan States. Under what name it was written or the title shall never be revealed. Even [my innocent?] children have never seen it. My second book, 'The Man Outside' received the fleeting glory of a best seller..."
The article also notes that Martyn was married to an accomplished musician and had two children.
Martyn's sojourn to Boston may have had some connection with the sale of his book to Boston-based publisher Little, Brown who published two of his books in 1912-14. However, Martyn had a letter printed in Time magazine on December 27, 1943, where it was noted that Martyn was the editor of Pearson's Magazine in 1915. So it would seem that he had returned to New York/New Jersey by then. Also worth noting is that Arthur W. Little was the credited editor of Pearson's until Frank Harris took over (with the September 1916 issue). J. J. Little remained the publisher until the title folded in April 1925.
According to his registration card he was living at Windermere Avenue, Interlaken, Monmouth County, New Jersey, in September 1918. At that time he was working as an editor for Warner Pub. Co. This card gives his date of birth as July 6, 1875, and his next of kin as Amy Martyn.
Wyndham Martyn appears in the 1920 US census living at 210 Windermere Avenue (the search facility of Ancestry.com is terrible when it comes to hand-written records which is why I didn't find him until Victor Berch pointed out where to look). It records the author as being 45 years of age, his wife (Elizabeth) was 39, and he had two daughters, Phylis (16) and Cynthia (13). Phylis was born in England (c.1903) whilst Cynthia was born in New Jersey (c.1906). Cynthia can also be found in the 1930 census -- 23-year-old Cynthia V. Martyn, lodging in Santa Monica where she worked as a teacher in a private school. Phylis was, I believe born Phyllis Barbara Hosken (see above).
Immigration records from 1933, previously cited, note that Wyndham and Amy Martyn were living in Santa Monica by that time. The letter from Wyndham printed in Time magazine (1943) was also from Santa Monica.
Victor, again, used Martyn's long connection with California to come up with a strong suspect in California's death records which list one Grenvil W. Martyn, born in England on July 6, 1874, who died on March 15, 1963, in Los Angeles. His mother's maiden name was listed as Camplin.
What can we conclude from all this? First, that Wyndham Martyn was flexible with his name. He writes as W. H. G. Wyndham Martyn and as William Grenvil. His death is registered as Grenvil W. Martyn. We've already established that his mother's maiden name was not Camplyn (with a 'y' and there is no Clarice Camplin (with an 'i') mentioned in any available birth/death/marriage or census record. The W. H. originally used for writing seems to me to be a clue that he was born William Henry M. Hosken (whose mother's maiden name was Campling). Backing this up is the fact that Hosken was born in the third quarter of 1874 (which ties in with his birth date as registered at the time of his death; note, too, that both Grenvil W. Martyn and Wyndham Martyn were born on July 6). William Hosken was married to an Amy Elizabeth Stanley, as was "Wyndham Martyn", there was a Phyllis Hosken born in the UK at the time when "Phylis Martyn" is said to be born.
Remember also Hosken's literary relatives: Ernest Charles Heath Hosken, William's cousin, was a writer (as Heath Hosken) and John Herrington has pointed out that Ernest's younger brother, Clifford James Wheeler Hosken (1882-1950), was also a writer under his own name and the pen-name Richard Keverne.
From Martyn's own admission, we know that he had some financial troubles (which he blames on "a defaulting lawyer and a bank that failed") which may have caused him to flee the country under a false name. So far no records of him leaving the UK or arriving in America have been found, but I think the evidence is stacking up that Wyndham Martyn was really William Hosken.
Novels (series: Christopher Bond; Anthony Trent)
The Man Outside, illus. C. M. Relyea (serial as 'John Paget's Progress'). New York, Dodd, Mead & Co., 1910.
All the World to Nothing, illus. H. H. Leonard. Boston, Little, Brown & Co., 1912; London, Sampson Low, 1913.
Under Cover by Roi Cooper Megrue, novelized by Wyndham Martyn, illus. William Kirkpatrick. Boston, Little, Brown & Co., 1914; London, Jarrolds, 1917.
Anthony Trent, Master Criminal (Trent). New York, Moffatt, Yard & Co., 1918; London, Herbert Jenkins, 1922 ; abridged, , Dublin, Mellifont Press, 1942.
The Secret of the Silver Car (Trent). New York, Moffatt, Yard & Co., 1920; London, Herbert Jenkins, 1922.
The Mysterious Mr. Garland (Trent). London, Herbert Jenkins, 1923 .
The Return of Anthony Trent (Trent). London, Herbert Jenkins, 1923; New York & Newark, N.J., Barse & Hopkins, c.1925.
The Bathurst Complex. London, Herbert Jenkins, 1924; as The Murder in Beacon Street, New York, R. M. McBride & Co., 1930.
The Recluse of Fifth Avenue. London, Herbert Jenkins, 1925; New York, R. M. McBride & Co., 1929.
Trent of the Lone Hand (Trent). London, Herbert Jenkins, 1927.
Anthony Trent: Avenger (Trent). London, Herbert Jenkins, 1928.
The Triumphant Prodigal. London, Herbert Jenkins, 1928.
The Death Fear (Trent). London, Herbert Jenkins, 1929; New York, R. M. McBride & Co., 1929.
Murder Island (Trent). New York, R. M. McBride & Co., 1928; London, Herbert Jenkins, 1929.
The Social Storming. London, Herbert Jenkins, 1930.
The Trent Trail (Trent). London, Herbert Jenkins, 1930; New York, R. M. McBride & Co., 1930.
The Scarlett Murder (Trent). London, Herbert Jenkins, 1931.
Christopher Bond, Adventurer (Bond). London, Herbert Jenkins, 1933.
The Great Ling Plot (Trent). London, Herbert Jenkins, 1933.
Death by the Lake (Trent). London, Herbert Jenkins, 1934.
The Spies of Peace (Bond). London, Herbert Jenkins, 1934.
Criminals All (Trent). London, Herbert Jenkins, 1935.
Nightmare Castle (Trent). London, Herbert Jenkins, 1935.
The Denmede Mystery (Bond). London, Herbert Jenkins, 1936.
The House of Secrets (Trent). London, Herbert Jenkins, 1936.
The Blue Ridge Crime (Trent). London, Herbert Jenkins, 1937.
The Old Manor Crime (Trent). London, Herbert Jenkins, 1937.
The Marrowby Myth (Bond). London, Herbert Jenkins, 1938.
Murder Walks the Deck (Trent). London, Herbert Jenkins, 1938.
Noonday Devils (Bond). London, Herbert Jenkins, 1939.
Trent Fights Again (Trent). London, Herbert Jenkins, 1939.
Capture (Bond). London, Herbert Jenkins, 1940.
The Ghost City Killings (Trent). London, Herbert Jenkins, 1940.
The Headland House Affair (Trent). London, Herbert Jenkins, 1941.
Shadow Agent (Bond). London, Herbert Jenkins, 1941.
Men Without Faces (Trent). London, Herbert Jenkins, 1943.
Cairo Crisis (Bond). London, Herbert Jenkins, 1945.
The Last Scourge (Trent). London, Herbert Jenkins, 1946.
Stones of Enchantment (Trent). London, Herbert Jenkins, 1948.
Manhunt in Murder (Trent). London, Herbert Jenkins, 1950; New York, Roy Publishers, 1958?.
The Chromium Cat (Bond). London, Jenkins, 1952.
Novels as William Grenvil
The Mutiny of the Albatross. London, Herbert Jenkins, 1923.
The Man from the Desert, and other stories. London, Herbert Jenkins, 1924.
Short Stories as Croydon Heath
The Terror of the Rats (The Thrill Book, 15 Aug-1 Sep 1919)
Dregs of Fate (Action Stories, Sep 1921; reprinted Hutchinson's Adventure-Story Magazine, Oct 1922)
Short Stories as W. H. G. Wyndham
The Conversion of Dad (Pearson's Magazine (US), Apr 1906)
The Fairies and the Babies (poem; Pearson's Magazine (US), Apr 1906)
The Triumph of the Twins (Pearson's Magazine (US), May 1906)
The Capture of the Ideal (Pearson's Magazine (US), Jun 1906)
When the Toys Wake Up (Pearson's Magazine (US), Sep 1906)
The Fight for the Black Box (Pearson's Magazine (US), Jun 1907)
The Man Who Was Afraid (Pearson's Magazine (US), Jul 1907)
The Sunset Lady (poem; Pearson's Magazine (US), Jul 1907)
By Mere Chance (Pearson's Magazine (US), Aug 1907)
The Millionaire Who Couldn't Ride (Pearson's Magazine (US), Oct 1907)
Some Reflections of a Serious Child (poem; Pearson's Magazine (US), Dec 1908)
Short Stories as Wyndham Martyn
The Amorous Burglar (Pearson's Magazine (US), Jan 1913)
Extra Talent (Pearson's Magazine (US), Jun 1915)
The Weather Vane (Pearson's Magazine (US), Nov 1915)
Bill Snyder, Elephant Man (Pearson's Magazine (US), Feb 1916)
Finger Prints adn Inspector Faurot (Pearson's Magazine (US), Mar 1916)
Oscar of the Waldorf (Pearson's Magazine (US), Apr 1916)
Elsie Ferguson, Actress (Pearson's Magazine (US), May 1916)
Wonderful New York (article; Pearson's Magazine (US), Jun 1916)
Concerning Clowns (Pearson's Magazine (US), Jul 1916)
Factors (The Smart Set, Nov 1916)
The Old Marquis (The Smart Set, Nov 1916)
Celestial Crowns (The Smart Set, Dec 1916)
Youth's Call (The Smart Set, Jan 1917)
Lesbia's Eyes (The Smart Set, Feb 1917)
E.D. (The Smart Set, Mar 1917)
The Constant Pilgrim (The Smart Set, Sep 1917)
Trial Balances and the One Girl (People's Favourite Magazine, Oct 1919)
Samuel Perkins, Unable Mariner (Everybody's Magazine, Apr 1921)
The Man from the Desert (Action Stories, Sep 1921)
Larry's Last Job (Action Stories, Oct 1921)
The Garden of Tortures (Action Stories, Feb 1922)
Behind the Walls of Fog, with Henry Noel Potter (Action Stories, Apr 1922)
Life, Liberty and -- (Everybody's Magazine, Dec 1922)
The Bathurst Complex (serial; Munsey's, May-Sep 1923)
Garland the Great (Argosy All-Story Weekly, 16 Jun 1923)
Desert Dust (Illustrated Novelets, Mar 1924)
London Larry's Last Coup (Triple-X, Nov 1924)
What the Gray House Hid (serial; Munsey's, Jan-Mar 1925)
The Baited Trap (serial; Flynn's, 11 Apr-2 May 1925)
The Recluse of Fifth Avenue (Everybody's Magazine, May 1925)
The Silent Sentence (Clues, Jan 1927)
The Fourth Sentence (Hutchinson's Adventure & Mystery Story Magazine, Jun 1928)
The Sporting Chance (Cosmopolitan, Jan 1930)
The Ju Ju Diamond (Jungle Stories, Sum 1942; reprinted Jungle Stories, Win 1951)
The River of Death (Action Stories, Aug 1942)
White Man's Code (Action Stories, Fall 1942)
The Two Rogues (Jungle Stories, Win 1942)
The White Gorilla-King (Jungle Stories, Apr 1943)
Lutembe the Avenger (Jungle Stories, Win 1943)
The Abstractor (Ellery Queen's Mystery Magazine, Aug 1952)
Caught with the Goods (1914) (story)
Number 99 (1920) (based on the story 'One Week-End')
The Star Reporter (1921) (based on the novel The Mysterious Mr. Garland)
The Silver Car (1921) (story as Wyndham Martin, based on the novel The Secret of the Silver Car)
Dangerous Pastime (1922) (story as Wyndham Martin)
The Price of Youth (1922) (story as Wyndham Martin)
Pawns and Queens (1927) (story as by Windham Martyn)
All the World to Nothing (1916, screenplay by Stephen Fox)
AMY ELIZABETH MARTYN -- ADDITIONAL NOTES
Amy Elizabeth Stanley was born in Kensington in 1876 and was living with her widowed mother, Elizabeth, and younger brother, 3-year-old Frederick, in 1881 in Montague Road, Wimbledon. Elizabeth (born in Edinburgh, Scotland) was only 27. There's a Frederick registered as born in Kensington in 1877 which would fit the bill.
I'm taking a wild stab in the dark and will say that her father might be Frederick Stanley who died in Kensington in 1877, aged 34. I would also say that Mrs. Stanley was formerly Elizabeth Groves and the two were married in Kensington in the 2nd quarter 1876. So she was already pregnant with Amy, who was born in the 3rd quarter of the same year.
I believe that, in 1891, Frederick is at East Lynne -- a college? -- at Portsea, Portsmouth, which, if you recall, is where James J. Hosken and his family lived -- indeed, William Henry Hosken is listed as a medical student living with his uncle at 4 Gordon Crescent, Portsea, that same year. He's only 3 years older than Frederick so there's a chance that the two knew each other and it was through her younger brother that William met Amy!
Amy, meanwhile, is a student at Parkfield College in Chipping Norton, Herts., in 1891. Amy married William Henry Hosken in 1900 and, I now believe, subsequently changed her name to Amy Elizabeth Martyn.
Amy (under the name Elizabeth Martyn) and her daughter Phyllis are registered as arriving in New York in 1905, their last permanent address being in Bruges, Belgium. They sailed from Antwerp on October 14 and arrived on October 24 where it was noted that this was their first time in the United States. They were joining W. H. Martyn whose address was given as 88(?) E. 23rd Street, New York. At the moment, it's unknown when Martyn himself travelled to the USA but there was a William Hosken who arrived in New York on October 28, 1904 (but who gave his age as 28 years, 4 months, implying he was born in mid-1876 rather than mid-1874).
THE MARTYN CHILDREN -- ADDITIONAL NOTES
From various passenger manifests and official records, it would seem that Phyllis and Cynthia both travelled quite extensively with their mother in their early years. From various sources we have gleaned the following:
Phyllis Barbara Martyn was born on December 12, 1903 and subsequently married Oswald Porter of Santa Monica. She died on June 8, 1983, in Inyo, California.
Cynthia Vivien Martyn, born October 7, 1906 in New Jersey, was also married and her death, on February 10, 1990, is registered in Los Angeles under the name Cynthia Vivien Evans.