Mercure was born Monique Émond in Montreal, Quebec. She was born to Eugene and Yvonne (née Williams) Emond. Her parents enrolled her as a young child in diction, tap dancing, musical theory and cello classes. She married Pierre Mercure in 1949; the couple had three children.
She studied music and dance, before sampling the theater in the company of St. Lawrence College. In 1960 she held her first major role in replacing an Actress in The Threepenny Opera. In 1992 she won a Genie Award for her role as Fadela in Naked Lunch. In 1979, she was made an Officer of the Order of Canada and was promoted to Companion in 1993. In 1999, she won another Genie for her role as Grace Gallagher, an elderly resident in Conquest set in Conquest, Saskatchewan.