After karting, Grenier moved to Formula BMW Americas in 2008 with Apex-HBR Racing Team, finishing fourth in points with a pole at Road America and six podium finishes. In 2009 he made six starts in Star Mazda for Andersen Racing, good enough for 18th in points. In 2010 he drove the full Star Mazda schedule for Andersen and finished eighth in points, finishing second from the pole at O'Reilly Raceway Park at Indianapolis and winning from the pole in race one at Autobahn Country Club.[1]
He returned to racing in 2016 when he signed to drive in the Porsche Carrera Cup Italia with Tsunami Racing. After a 3rd-place finished in his first outing at Monza he consistently made top 5 with a few podium finishes. The last race of the season was the turning point of his season with a win at Mugello in damp conditions. This secured 4th place in the driver Championship on his return to racing after a 2-year absence.
For the 2023 season, Grenier joined Team Korthoff Motorsports in the IMSA SportsCar Championship, driving in the GTD class alongside Mike Skeen.[3] He was also added to the Mercedes-AMG factory roster for the 2023 season.[4]