Persistence in high school curricula leading to science, technology, engineering, and mathematics (STEM) careers is structured by complex institutional systems, but developmental processes underlie how young people navigate these systems. This study examined differences in the development of STEM identity and efficacy during high school among Mexican-origin youth--a large and fast-growing demographic group that shows developmental assets and risks. Contextualizing development within larger community structures, this examination focused on the diverse array of destinations throughout the United States where Mexican-origin youth are living as contexts for their STEM identity and efficacy development. Drawing on a dataset integrating the High School Longitudinal study of 2009; Civil Rights Data Collection, decennial U.S. censuses, and the American Community Survey, multilevel models revealed variability in Mexican-origin math/science identity and efficacy development across destinations. Mexican-origin youth in established destinations had higher net growth in math identity but lower net growth in science efficacy than Whites in established destinations. Mexican-origin youth in new destinations followed similar trajectories as their Mexican-origin peers in established destinations but had lower net growth in science identity. Additionally, these patterns varied by immigrant generation. Mexican-origin youth who were the U.S.-born children of immigrants in established destinations had higher net growth in math identity than Whites in established destinations, but this generational group in new destinations had lower net growth in math identity, science identity, and science efficacy than these peers. These findings highlight the importance of communities and their embedded ecological contexts in shaping STEM identity and efficacy among Mexican-origin youth.