Yes, you can use American dollars in Mexico in most border areas (like Tijuana) and major tourist destinations (like Cancun, Cabo San Lucas, Mazatlan). When you get further inland it’s not so easy, but you can change US dollars into Pesos at any bank in any town/city. ATMs are generally easy to find in larger towns and cities.

Be careful when shopping with US dollars, as not every vendor will give you a good rate of exchange.