Using data from a longitudinal study of Mexican-origin girls (ages 11-17 at Time 1), we examined sociocultural (i.e., family structure, nativity, and acculturation), interpersonal (i.e., supportive parenting and conflict), and developmental (i.e., menarche timing and autonomy expectations) predictors of sexual initiation. Using Cox proportional hazards models, we explored whether Time 1 variables predicted the occurrence and timing of first sexual intercourse reported 2.5 and 3.5 years later. Results indicated that the likelihood of early sexual intercourse was higher among first-generation than second-generation immigrants. In addition, living with a stepparent (compared with two biological parents) was associated with a higher likelihood of early intercourse. Furthermore, early autonomy expectations emerged as a salient predictor of intercourse, such that girls with earlier autonomy expectations were more likely to have earlier intercourse than girls with later autonomy expectations. Taken together, results highlight the importance of considering developmental and contextual factors when studying Mexican-origin girls' sexual initiation.