Drafted by the Colorado Avalanche 45th overall in the 1999 NHL Entry Draft, Grenier was traded to the Boston Bruins along with Brian Rolston, Samuel Pahlsson and a 2000 1st round draft pick for Ray Bourque and Dave Andreychuk on March 6, 2000. Left unsigned by Boston, Grenier signed with the Phoenix Coyotes prior to the 2001–02 season. Since beginning his pro career, he has spent most of his time in the AHL with a few callups to the NHL with the Coyotes, Vancouver Canucks, and Philadelphia Flyers.