https://www.quora.com/Can-I-drive-a-US-registered-car-in-Ontario-Canada
Depends on the driver’s residency status in Canada. Strictly speaking, a Canadian resident is not permitted to drive a US registered vehicle in Canada which has not been properly imported by the owner. This is a safety, customs and excise issue; Canada wants to ensure that vehicles that are being operated on Canadian roads meet Canadian safety standards and that import duties and taxes have been paid. That said, the circumstances will matter on whether the law is enforced should a driver be stopped by the police or at the border.
US residents temporarily in Canada may operate their US registered vehicle while they are in Canada without issue for up to 12 months provided they have adequate third party liability coverage. The Ontario minimum requirement is $200,000 CAD; much higher than most US state’s minimum coverage requirements.