Unless you're referring to Hawaii, Puerto Rico, or any other US island not connected to the continental US with a bridge, the answer is yes, you can drive to Ontario, Canada, or even Ontario, California.
No driving necessary. Ontario is a part of the USA and has been since its incorporation as a city in 1891. The land on which it was founded has been American territory since the Treaty of Guadalupe Hidalgo in 1848.
UPDATE: /trolling. Yes you can. You don't even need a passport.