Developmental systems theory and life span development describe the role of individual--context interactions in individual development but have not directly addressed how individuals pursue achievement goals in institutional contexts. We developed a theory informed by these perspectives that explains how institutional contexts affect emerging adults' success in transitioning to and progressing through college. We theorize that institutional contexts increase individuals' probability of attaining specific goals when they provide stronger channels that offer more versus fewer structural supports for these goals. Moreover, we theorize, these institutional channels influence which individual differences, including belonging certainty, growth mindset of intelligence, and grit, will be useful in goal pursuit, above and beyond individuals' academic preparation and demographic factors. We examined postsecondary goal pursuit over a 6-year period among 1,850 students who attended one of four district high schools in Pennsylvania or Massachusetts. On average, they began the study at 17.91 years of age; 48% were male, 43% belonged to a historically marginalized ethnicity, and 56% had free or reduced lunch status. We found that channel strength and psychological factors interacted in ways predicted by our theory. Higher belonging certainty and growth mindset in Grade 12, which encourage a focus on process and progress, predicted better outcomes in weaker postsecondary channels, especially college enrollment and on-track progress. Higher grit, which encourages a commitment to goal attainment, predicted better outcomes in stronger postsecondary channels, especially on-time graduation. The study locates the importance of psychological factors in predicting goal attainment in different-strength institutional channels during emerging adulthood.