Tax residency
You become a Mexican tax resident if Mexico is your center of vital interests OR you spend more than half the year in Mexico. Holding a Temporary Resident card alone does NOT make you a tax resident.
Income tax rates
Mexican personal income tax is progressive, from about 1.92% to 35% on the highest brackets.
Foreign income
Mexican tax residents are taxed on worldwide income. Mexico has tax treaties with most countries to avoid double taxation.
US citizens
US citizens always file with the IRS. The Foreign Earned Income Exclusion (FEIE) lets you exclude up to ~$120,000 of foreign-earned income from US tax. Use a cross-border accountant.
Filing
Mexican tax year is the calendar year. Annual returns are due in April.
This is a general overview, NOT personal tax advice. Always consult a qualified accountant familiar with cross-border taxation before making decisions.