Jaguars used to be found as far north as the Grand Canyon, and their historical range may have extended as far north as Oregon and as far east as North Carolina.
For more info, check out the Northern Jaguar Project, which is a conservancy that is trying preserve jaguars in northern Mexico and the United States.
Jaguars are the only true big cats in the Americas. All true big cats cats are either in the genus Panthera or are closely related to that genus. The closest relative of the cougar is the jaguarundi, which is a very small cat, so cougars are technically not big cats.