Despite abundant research on the benefits of dispositional mindfulness and mindfulness-based interventions, little is known about the naturally occurring developmental course of mindful awareness. Given dramatic strides during adolescence in competencies that may support mindfulness (e.g., emotion regulation), the high school years are a suitable window to study developmental changes in mindfulness. Using an accelerated cohort design to examine the longitudinal trajectory of mindfulness from 9th grade through 12th grade, we employed data from a 5-year study of 3,453 U.S. adolescents (55% female; 40% Latinx, 38% White, 13% Black, and 7% Asian) from high schools in three geographical regions in the U.S. Adolescents annually completed self-report surveys measuring mindful awareness and several internal and external factors. Latent growth curve analyses found that, after controlling for cohort, there was no average linear change in mindful awareness from grades 9 through 12, although there was significant interindividual variability in slopes. Within-person analyses found that in grades when needs-supportive climate was higher and discrimination was lower, adolescents reported greater mindful awareness. Unexpectedly, in grades when perspective taking and prosocial behaviors were higher, mindfulness was lower. These findings inform developmental understanding of mindful awareness and the ways that it naturally occurs and is supported during adolescence.