This research article examined whether temporal fluctuations in context ethnic diversity account for current levels of implicit ethnic-American associations. Temporal fluctuations in ethnic diversity at the metropolitan level were assessed using data from four decennial U.S. censuses (1980–2010) and distinguishing three dimensions of context ethnic diversity (minority representation, variety, and integration). Project Implicit data (2011–2017) indexed the extent to which American identity was implicitly associated with European Americans over Asian Americans (i.e., American = White associations). Data were analyzed using multilevel modeling (N = 152,011, nested within 226 metropolitan areas). Steeper increases in the proportion of Asian Americans were related to weaker implicit (but stronger explicit) American = White associations. Increases in ethnic integration accounted for stronger implicit American = White associations when integration fluctuations reflected accelerating rather than decelerating trends. These results suggest that current levels of implicit ethnic-national associations are linked to complex patterns of ethnic diversity fluctuations.