Ethel Seath was born in 1879 in Montreal. Due to the breakup of her parent's marriage, Seath helped support her four siblings, and as early as 16, she began working as an illustrator for the Montreal Witness. Circa 1898 she studied at the Conseil des Arts et Manufactures, taking drawing with Edmond Dyonnet and lithography with Robert Harris. In 1901 she joined the Montreal Star as an illustrator, followed by the Family Herald. After 20 years working as an illustrator, Seath was appointed art teacher in 1917 at Montreal's The Study, a private girl's school. She also taught children's art classes at the Art Association of Montreal from 1937 to 1941. Teaching art was a life-long committment, and after The Study's founder Margaret Gascoignes - a close friend - died in 1934, Seath continued teaching there until retiring in 1962.

Seath was a member of Montreal's Beaver Hall Group formed in 1920, and after its formal demise, continued her ties with her fellow women members, exhibiting with them in group shows at the Art Gallery of Toronto in 1940, the Art Association of Montreal in 1944, the Willistead Art Gallery in 1945 and the Montreal Museum of Fine Arts in 1940.

Although she worked, Seath painted and actively exhibited throughout her life. Her subjects were cityscapes, still lifes and landscapes, depicted using curvilinear patterns, bold masses of colour and simplified detail. She participated in the Art Association of Montreal's spring exhibitions as early as 1905, and in the Royal Canadian Academy Annual in 1906. From 1931 to 1933 she participated in the Annual Exhibition of Canadian Art at the National Gallery of Canada. Her work was included in the important 1925 British Empire Exhibition in 1925 at Wembley, England and A Century of Canadian Art at the Tate Gallery in London in 1938. She was elected to the Canadian Group of Painters in 1939 and exhibited in their group shows, and joined the Contemporary Art Society circa 1940.

Seath died in Montreal in 1963