The incident occurred Tuesday at around 12:30 p.m. at the Atlantis Athletics and Fitness Centre at Mountainview Road and Guelph Street, Halton Regional Police said in a release.
A Ford Expedition SUV smashed through the glass window, "demolished the reception area and nearly came out through the rear of the building," according to the release.
A 36-year-old employee of the gym was pushed though the foyer, through a wall into the rear of the building, where she was pinned underneath the vehicle when it finally stopped.
The Halton Hills fire department was able to get the woman, who is from Georgetown, out from under the car after about 45 minutes. She was rushed to Sunnybrook Health Sciences Centre in Toronto, where she received treatment for serious injuries, police said.
Police are speaking to the 33-year-old driver of the SUV, who is cooperating in the investigation and has provided a statement. Both the driver and her 19-month-old son were not injured.
Investigators are now reconstructing the crash.
"The focus of the investigation will be on the mechanical fitness of the SUV and/or driver error," police said.
No one is allowed to enter the building, in which a bank, federal government offices and several businesses are located, until it is deemed safe by structural engineers.