Migration Policy InstituteSince claiming the top spot among all immigrant groups in the 1980 census, the Mexican immigrant population in the United States has continued to outsize other immigrant groups. In 2008, there were 11.4 million Mexican immigrants in the United States, accounting for 30.1 percent of all U.S. immigrants and 10 percent of all Mexicans. Over half of all Mexican immigrants reside in the United States illegally.