Born in Hampstead in 1922, the daughter of Albert Edward Elsy (a court photographer) and his wife Ethel Jeffrey (nee Forster), she was educated at Lymington House School, Hampstead, and Kingsley School for Girls, Hampstead, before attending Oakley Training College for Teachers, Cheltenham.
She was a teacher at state schools in Littlebury, Essex (1947-48), and London (1948-49) before teaching at a private school in Hampstead (1949-51). She worked a variety of odd jobs and as a freelance writer during the 1950s whilst travelling extensively throughout Europe.
She was associated with the Realist Film Unit (1957-58) and an assistant to script writer at Associated Rediffusion (1958-59), before becoming a sub-editor on the Children's Newspaper in 1960-62. She subsequently worked as an editorial assistant for B.P.C. Publishing (19563-64) and Evans Bros. (1965-66) before becoming children's book editor for Abelard-Schuman in 1967-68. From 1968, she worked as a freelance writer
She contributed to the Sunday Telegraph, Christian Science Monitor, Travel, In Britain, My House & Family, Elizabethan, Nursery World, Odhams Children's Encyclopedia, Observer, Voyager, She, Art & Antique, Illustrated London News and the the BBC.
For the latter she wrote a number of children's TV shows, including The Bus That Wouldn't Budge (17 March 1954), The Bed That Ran Away (Welsh Regional, 26 May 1955), The Adventures of Hetty the Hat (Welsh Home Service, 1 December 1955).
Elsy commented to Contemporary Authors:
My parents' business/home stood opposite the main Hampstead Public Library, which was very convenient for me when a child, as I was a great reader ... I started writing when a teenager, gave it up for a period, and started again when I was a teacher. I wrote a large number of children's stories and tried my hand at novels, but unfortunately for me, it was a bad period, because it was not long after the war and there was a shortage of paper; there were also few magazines to sell them to. I suppose writing eventually became a habit, and/or it was necessary for me to have some form of self expression.Mary Elsy lived for many years in Finchley Road, Hampstead, and her funeral was held at Golders Green Crematorium on 3 April 2013.
My motivation is self-expression. I like to write, and would have liked to be an artist. I believe in internationalism and like people, but don't always practice what I preach. I often change my mind. I don't believe in dogma, but in pragmatism. I find life interesting, but am not very clear why I'm here.
I am not sure what sort of message, if any, I am trying to put over. Certain things, like hyprocrisy and injustice, make me angry, and I am somewhat cynical, but I would never presume to try to reform the human race.
Travels in Belgium and Luxembourg. London, Andre Deutsch, 1966.
Brittany and Normandy. London, Batsford, 1974.
Travels in Brittany, with Jill Norman. London, Merehurst Press, 1988.
Travels in Normandy, with Jill Norman. London, Merehurst Press, 1988.
Travels in Burgundy, with Jill Norman. London, Merehurst Press, 1989.
Travels in Alsace & Lorraine, with Jill Norman. London, Merehurst Press, 1989.
Pedals and Petticoats. On the road in post-war Europe. Chichester, Summersdale, 2005.