Mowat was born and educated in Toronto, Ontario. She graduated from Havergal College and the Ontario College of Art & Design as a Graphic designer and is married to author Farley Mowat. The couple split their time between Port Hope, Ontario, and the Cape Breton community of River Bourgeois, Nova Scotia.

Mowat began writing memoirs in the 1960s, leading in 1983 to her first book The Outport People, about the five years she and husband Farley spent at the start of their marriage in the outport Newfoundland community of Burgeo. Her second book Pomp and Circumstances (1989) arose from witnessing protocol behind the scenes at the Governor General's residence Rideau Hall in Ottawa. A 2005 memoir Travels with Farley: A Memoir describes the couple's life in the Magdalen Islands, following the still-born death of their only child. She has also written a trilogy of young-adult fiction: The Girl from Away (1992), The French Isles (1994) and Last Summer in Louisbourg (1998).