Emergency crews were called to the intersection of Wellington Road 16 and Highway 89 just before midnight.
OPP said the driver, who was alone in the vehicle, was taken to a local hospital and later died.
The identity of the driver has not been provided. Police also did not comment on any possible factors into the crash.
Highway 89 was closed between Southgate Sideroad 13 and Southgate Sideroad for several hours for the investigation.
Mount Forest is approximately 65 kilometres northwest of Guelph on Highway 6.
Any witnesses to the crash or anyone with information is asked to call Wellington County OPP at 1-888-310-1122.