Marriage to a U.S. citizen does not confer automatic citizenship. A Mexican national (or any foreign spouse) must navigate a structured legal pathway.
File Form I-130 (Petition for Alien Relative) and adjust status (if in the U.S.) or consular process (if abroad). Conditional residence (2 years) applies if marriage is under 2 years old at approval; file Form I-751 to remove conditions.
After 3 years as a permanent resident (if still married to and living with the U.S. citizen spouse), the spouse may apply for citizenship via Form N-400. Standard requirement is 5 years; the 3-year exception requires continuous marital union.
Legal counsel is advisable to navigate potential complexities (prior immigration violations, criminal history, etc.).