Generally speaking, crossing back to the U.S. is a breeze with a dog or cat. It's important to note that there are federal and state regulations and it's important to know both.
In an nutshell, to return to California only dogs, not cats, require a rabies certification. Health certificates are not required for either dogs or cats.
As information can change, below are some of the regulations with links to find the latest information:
Federal level: According to the CDC, all dogs and cats must appear healthy on arrival. The CDC does not require rabies certificates if the dog is coming from Mexico. They state as an example: "Your adult dog lived in the United States and visited Mexico. This dog does NOT require a rabies certificate or CDC Dog Import Permit, because Mexico is NOT on the list of high-risk countries for dog rabies." View more federal information about bringing dogs and cats.
State level: The USDA has a handy tool to check state requirements to import a pet. Scroll down to "Additional State Requirements" and select the U.S. state you will be crossing into from Mexico. For example, for California, the CDFA states:
Consider bringing proof of pet ownership with you as it may come in handy.
Generally speaking, dogs and cats will NOT be quarantined as long as they are healthy and have the required paperwork to enter the U.S. However, according to the Pets and Wildlife Division of U.S. Customs, all about Bringing an Animal into the United States.
Thank goodness, some good news! There is no "duty" or fee for dogs or cats re-entering the U.S.
According to the CDC, the youngest a puppy can be to legally enter the US is 4 months old. Puppies can not get vaccinated before 3 months of age, and once vaccinated they need to wait 30 days before traveling so the vaccine can take effect.
According to the CDC: Adult dogs older than 15 months of age that have previously received a rabies vaccination given no earlier than 3 months of age and that has since expired may be imported immediately following booster vaccination, without the need to wait for 30 days.
Dogs that have never received a rabies vaccination before must be vaccinated at least 30 days prior to arrival in the U.S.
Importation of dogs and cats into the U.S. is regulated by the Center for Disease Control (CDC). According to the CDC, all domestic cats and dogs must show no evidence of disease that can be communicable to humans at the port of entry.
Even if all of your paperwork is in order, if your dog or cat does not look like it is in good health, further examination by a licensed vet (at your expense!) will be necessary before your dog or cat is allowed to re-enter the U.S.!
Center for Disease Control