Mabel Killam Day was born in Yarmouth in 1884, the daughter of a noted family of local shipbuilders. She studied at the Mount Allison Ladies' College (1900-1904) under John Hammond and with Robert Henri in New York (1905-1909) where she studied along side of Edward Hopper and George Bellows.
With her husband, the writer Frank Parker Day, she lived in the eastern United States and England, returning to Nova Scotia in 1933, due to her husband's poor health.
Day had a successful artistic career in the United States. She began exhibiting in New York in 1909 and had her first solo exhibition in 1923 (Carnegie Institute, Pittsburgh). In 1913 her work was shown with the Associated Artists of Pittsburgh where she won first prize for a painting of the St. John River. Her paintings were also exhibited at the Pennsylvania Academy of Fine Arts, the Art Institute of Chicago and the Montreal Museum of Fine Arts. She was a Director of the Associated Artists of Pittsburgh (1913).
After her return to Yarmouth, she exhibited regularly with the Nova Scotia Society of Artists and the Maritime Art Association. She was president of the Yarmouth Art Society from 1957 to 1960, and was an art teacher in the Adult Education Department in Yarmouth, 1947-1959.
Much of her earliest and most important work was destroyed in a 1937 fire at Zwicker’s Gallery, just prior to the opening of a solo exhibition.