In 1974, Mackay published her first book, Mini-Bike Hero (Scholastic Canada), inspired by her son’s obsession with the sport. She went on to write nine more fiction and non-fiction titles, working with Scholastic Canada, Annick Press, Tundra Books, Kids Can Press, and James Lorimer.
In 1982, Mackay and co-author Marsha Hewitt received the Ruth Schwartz Award for their YA novel One Proud Summer (Women’s Press). The following year Mackay received the Vicki Metcalf Award, presented for an outstanding body of work in children’s literature.
In 1977, Mackay, along with Jean Little and nine other artists working in the field, founded the Canadian Society of Children’s Authors Illustrators and Performers. CANSCAIP now boasts more than 400 professional and 600 associate members across Canada.