Born in East Orange in 1903, New Jersey, Dorothy Duncan spent her formative years in the United States. She grew up in Wilmette, Illinois, with a Christian Science background. She received a Bachelor of Science in Botany from Northwestern University in Chicago in 1925 and worked as a journalist in the Chicago area for a number of years. In 1932, she made a trip abroad and on the return from England aboard the S.S. Penfield, she met Canadian Hugh McLennan. They fell in love, but McLennan’s father insisted that he should be financially independent before marrying. As a result, he wound up spending three unhappy years at Princeton doing graduate studies in Classics before he could look for a job, which was made more difficult by the Great Depression. They married in 1936 at her home in Wilmette, Illinois, and settled in Montreal, where MacLennan found work teaching at Lower Canada College. While Duncan was often addressed as Dorothy MacLennan in her correspondence, she published under her maiden name.

Duncan’s published works include “You Can Live in An Apartment” (1939), “Here's to Canada!” (1941), and “Bluenose: A Portrait of Nova Scotia” (1942). In 1944, she published a biography of the Czechoslovakian-Canadian Jan Rieger, entitled “Partner in Three Worlds,” for which she was awarded the 1946 Governor General’s award for creative non-fiction. Duncan was a member of the Authors’ League of America and the Canadian Authors’ Association. She was also the director of the North Hatley Library Association, the organization that ran the library in the small Quebec town where she and MacLennan summered.

Duncan encouraged MacLennan with his writing, and urged him to write about Canada rather than the United States or Europe, which were the settings for his first novels. While he had trouble finding publishers for those first books, he found success after following her advice with “Barometer Rising,” his first novel set in Canada. She often proofread his books. The character of Catherine Martell in Hugh MacLennan’s “The Watch That Ends The Night” is largely based on Dorothy Duncan.

In the late 1940s, Duncan’s health deteriorated, a result of rheumatic fever she had suffered during her youth. After her doctors advised her to stop writing, she took up painting, with some public success. Her health declined, however, and she died on Easter Day, April 22, 1957, two weeks before her solo gallery show was to open in Montreal.