Sarah Robertson was born in 1948 to a family that was part of Montreal's English establishment. She studied from 1909 to 1924 at the Art Association of Montreal under William Brymner and Maurice Cullen. In 1923 she won a scholarship to study from the Women's Art Society.

Robertson was a member of the Beaver Hall Group that formed in 1920, and continued to show with her fellow women members long after the group broke up - in 1934 with Prudence Heward and Isabel McLaughlin at Toronto's Hart House and in 1940 with Prudence Heward, Anne Savage and Ethel Seath at the Art Gallery of Toronto. She went on sketching trips with A.Y. Jackson (who had also been a member), Prudence Heward, Ethel Seath and Nora Collyer to the Laurentians, the Lower St. Lawrence and to Nova Scotia. In about 1929, she travelled to Bermuda with Collyer.

Robertson painted portraits, florals and still lifes, but her primary interest was in landscape, based on idyllic childhood experiences at her family's country house, and her sketching trips into the countryside. As she matured, her work exhibited Modernist tendencies, and became freer and bolder. Also, her style showed influences from the Group of Seven, and she was included in their 1928 exhibition. Her work was praised by Group members Jackson and Arthur Lismer.

She had her first show at the annual Royal Canadian Academy in 1922, and she participated in the Art Association of Montreal spring exhibitions from 1935 to 1942. In 1924 she was included in the 1924 Wembley exhibition in England. In 1933 she was a founding member of the Canadian Group of Painters and participated in their group shows. Robertson also exhibited in the United States, in group shows at the Yale University Art Gallery in 1944, the Museu Nacional de Belas Artes in Rio de Janeiro and the Riverside Museum in New York.

Illness shortened Robertson's life, and she died in Montreal in 1948 at 57. In 1951 a memorial exhibition was held of her work at the National Gallery of Canada .