The English-born Kay Tremblay began working with the George Ballachine Ballet at Theatre Royal in London, and soon found herself touring Europe and performing. In 1954, she married a Canadian, moved to Montreal, and continued to work in the theatre. She also appeared in some of CBC's early teleplays, as well as in various radio dramas.

Her first feature film was 1964's Phoebe. She played in various other movies and guested in TV series like The Beachcombers, Friday the 13th, and Street Legal in the 1970s and 1980s; but her on-screen career didn't receive a jolt until the 1990s, after appearing as Aunt Eliza on Road to Avonlea (she would reprise the role in Happy Christmas, Miss King in 1998), winning a Gemini for the role in 1997. She also appeared as Cousin Jessie in two episodes of Wind at My Back. Various credits throughout the 1990s include Remember Me (based on the Mary Higgins Clark novel), Love and Murder, The Marriage Bed, and Moonlight Becomes You. Her final on-screen appearances were 2000's Santa Who? and 2001's Who Is Cletis Tout?. Kay's last on-stage appearance was during the 2002 Stratford Festival.

The last years of her life were spent in a nursing home in the Toronto area, where she died in 2005 at the age of 91.