MacGregor was born Mary Esther Miller in
Rugby,
Ontario in 1872. Her parents were John Miller, a schoolteacher, and Mary Brown Johnston. Both parents were of Scottish ancestry. She was the eldest of five children. She attended school in
Edgar, Ontario, and the
Orillia Collegiate Institute. She received her teacher's certificate from the
Toronto Normal School in 1896. Beginning in 1899, she taught for seven years in
Orillia. She began writing in 1905. She contributed a column to
Teacher’s Monthly and then worked on the editorial staff of the
Presbyterian Church's Sunday School Publications. In 1906 when she tried to publish her first novel she learned that her own name was in use by another author (as Esther Miller), so she chose to write under the
pen name Marian Keith. In 1909, she married Donald MacGregor, who was one of the founding members of the
United Church of Canada. In 1924, she wrote a book called
A Gentleman Adventurer, which she based on the life of a neighbour, Thompson Smith. Canadian literary scholars have cited this as her best work. In many of her novels, MacGregor sought to portray her connection between religious thought and social conscience in order to improve life for the poor and uneducated. She also wrote about tensions produced between Scottish and Irish settlers and the effects of education and urbanization on rural society. Her ability to capture regional attitudes and her use of social anecdotes using a style characterized by zest and humour earned her a strong local following. She and Donald retired to a farm on
Georgian Bay, but her husband was injured in a train accident and died in 1946. She spent the rest of her life living in
Owen Sound, Ontario, with her sister and died there in 1961. ==Works==