Born in Ottawa, Ontario, Mosher attended fourteen different schools in Montreal, Toronto and Quebec City, graduating from the École des Beaux-arts in 1967. He famously won entrance to this fine arts college (now part of UQAM) by forging his high-school graduation certificate, which he called his most successful work. He then began working for The Montreal Star, moving to the Montreal Gazette in 1972.
Aislin's drawings have also appeared in numerous international publications, such as Punch, The Atlantic Monthly, Harper's, National Lampoon, Time, The Washington Star, The New York Times and the Canadian edition of The Reader's Digest. He is the author of 43 books.
Terry Mosher has had a long association with the Old Brewery Mission, Montreal's largest shelter for the homeless, and in 2001, was appointed to the institution's board of directors.
Mosher famously turned down shares in the board game Trivial Pursuit for which he provided the original artwork. The co-inventor Chris Haney gave Mosher a choice: $1,000 or shares. Mosher took the cash.
He and fellow Montreal cartoonist Serge Chapleau were the subject of a 2003 documentary film, Nothing Sacred, directed by Garry Beitel.
He is the recipient of two National Newspaper Awards and five individual prizes from the international Salon of Caricature. In 1985, Mosher became the youngest person ever to be inducted into the Canadian News Hall of Fame. In 2002 he was made an Officer of the Order of Canada. In 2007, he was awarded an honorary Doctor of Letters from McGill University. In 2012 he was inducted into the Canadian Cartoonist's Hall of Fame (aka The Giants of the North) in a ceremony in Toronto as part of the 8th Annual Doug Wright Awards for Canadian Cartooning. He received the Canadian Version of the Queen Elizabeth II Golden Jubilee Medal in 2002 and the Canadian Version of the Queen Elizabeth II Diamond Jubilee Medal in 2012.
In 1993, Mosher became the first artist to have his work denounced by a member of parliament (Robert Layton) in the House of Commons as "a crime against fundamental Canadian values of decency and mutual respect."
On 12 March 2010, he drew a cartoon depicting a woman in a niqab as being in a prison. One community leader pointed out that he had made similar cartoons about women wearing a hijab.