Mexico City is the densely populated, high-altitude capital of Mexico. Mexico City is the capital and largest city of Mexico and the most-populous city in North America. The 1521 capture Tenochtitlan and immediate founding of the Spanish capital Mexico City on its ruins was the beginning of a 300-year-long colonial era during. Mexico City, city and capital of Mexico, synonymous with the Federal District. The capital of Mexico, the country, is the Federal District (Distrito Federal, D.