A painter who enjoyed depicting everyday objects, Seath often painted still lifes and landscapes with an eye for abstract qualities. Her treatment of space was often decorative, with attention to how various forms interacted with each other. Due to familial financial constraints (her father was chronically ill), Seath began work as an illustrator for "The Montreal Witness" and later "The Montreal Star" soon after finishing high school. Her commercial success allowed her to enroll at the Art Association of Montreal and, in 1911, to join Maurice Cullen's sketching trips in the Quebec countryside. In 1917, she became an art teacher at The Study, a Montreal private girls' school, where she remained for 45 years.
A founding member of the Beaver Hall Group, Seath also belonged to the Canadian Group of Painters and exhibited frequently with both associations. Her work was also shown in Baltimore, Maryland (1931), the New York World's Fair (1939), Yale University, Cambridge, Massachusetts, and the Tate Gallery, London. She is represented in the collections of The National Gallery of Canada and the Montreal Museum of Fine Arts.