Between 1790 and 1952, naturalization was reserved primarily for “free white persons.” Asian immigrants were deemed non-white and racially ineligible for citizenship by legislation and the courts. European immigrants and, importantly, Mexican immigrants were considered white by law and eligible for naturalization. Yet, few Mexicans acquired US citizenship. By 1930, only 9 percent of Mexican men had naturalized, compared to 60 percent of southern and eastern Europeans and 80 percent of northern and western Europeans. If Mexicans were legally white, why did they rarely acquire citizenship in the early decades of the 20th century? We go beyond analyses focused on formal law or individual-level determinants to underscore the importance of region and non-white social status in influencing naturalization. Using 1930 US Census microfile data, we find that while individual characteristics (e.g., length of residence and literacy) explain some of the gulf in citizenship, the context of reception mattered nearly as much. Even if Mexicans were “white by law,” they were often judged non-white in practice, which significantly decreased their likelihood of naturalizing. Moreover, the more welcoming political and social climate of the Northeast and Midwest, where most European migrants lived, facilitated their acquisition of American citizenship.