Isabelle Blais was born in 1975 in Trois-Rivières, Quebec. She is a graduate of the Montreal campus of the Conservatoire de musique et d'art dramatique du Québec. She quickly charmed critics by her stage interpretation of Juliette in Romeo and Juliette, and in 2001, won their attention for her role in Soft Shell Man by André Turpin.

In 2002, she appeared in Confessions of a Dangerous Mind by George Clooney. In 2003, she was awarded a Jutra Award for "Best Supporting Actress" for her role in Quebec-Montreal by Ricardo Trogi. In the same year, appeared in The Barbarian Invasions, by Denys Arcand. In 2004, she appeared in a leading role in Les Aimants, by Yves Pelletier, for which she was nominated for a second Jutra Award, for Best Actress, as well as being nominated for a Genie Award for Best Performance by an Actress in a Leading Role. In 2007, she appeared in a supporting role in Sur la trace d'Igor Rizzi, by Noël Mitrani.

Isabelle Blais is also a singer in the Québécois rock group Caïman Fu.