Upvote:2
I am assuming that you are a U.S. citizen. If you are not a U.S. citizen, you must show proof that you have the legal right to live in the U.S. whenever you are crossing the border.
If you are a U.S. citizen, you must show proof of U.S. citizenship when you enter the U.S. from Mexico.
If you fly from Mexico to the U.S., you must have a U.S. passport to board the plane. You cannot use your paper driver's license at the airport in Mexico.
If you are crossing by land from Mexico into the U.S., you must show proof of U.S. citizenship. Showing a U.S. passport is the fastest way to enter the U.S. If you don't have a U.S. passport, the border officers (also known as CBP) will take more time to verify you are a U.S. citizen. Showing a U.S. birth certificate, certificate of naturalization, or other proof of U.S. citizenship will shorten your wait time at the border. Your paper driver's license might prove your identity, but it won't prove that you are a U.S. citizen.
You can get an emergency passport if you don't have one at the local U.S. embassy or consulate if you are in Mexico.
You can apply to get a U.S. passport quickly through the State Department if you are still in the U.S. Their travel.state.gov
website explains the process in more detail.
TLDR: Get a U.S. passport before you travel abroad. It makes travel easier.