Police were called to a Costco parking lot at 71 Colossus Dr., in the Highway 7 and Weston Road area, at 6:30 p.m.
A passerby had called 911, saying the dog was "in obvious distress," according to a news release.
The responding officers confirmed the 10-month-old dog was "lethargic, panting heavily" and without water. They were preparing to break the vehicle's windows when the owner showed up.
Police say the dog was looked over by municipal animal control officers and put in the back of a air-conditioned cruiser to cool off.
The animal was found to be showing signs of "distress and dehydration" and was subsequently seized for overnight monitoring.
Const. Andy Pattenden told CBC News the dog would likely be released back to the owner today.
The dog's owner, a 31-year-old Vaughan woman, was charged with causing unnecessary suffering to an animal. The accused is due to appear in court on July 24.