While there is extensive evidence that STEM careers can be important pathways for augmenting social mobility and for increasing individual prestige, many youth perceive a STEM trajectory as an unattractive option. In the USA, women and members of historically marginalized racial and ethnic groups continue to be underrepresented across STEM disciplines. One vehicle for generating and sustaining interest in STEM is providing youth long-term access to informal science education (ISE) institutions. Here, we incorporate triangulation methods, collecting and synthesizing both qualitative and quantitative data, to examine how participation in a longitudinal ISE out-of-school time (OST) program facilitated by the American Museum of Natural History (AMNH) impacted the STEM trajectories of 66 alumni. Findings revealed that 83.2% of alumni engaged in a STEM major, and 63.1% in a STEM career, the majority whom were females and/or members of historically underrepresented racial and ethnic groups. Based on interviews with a purposeful sample of 21 AMNH alumni, we identified four program design principles that contributed to persistence in STEM: (1) affording multiple opportunities to become practitioners of science; (2) providing exposure to and repeated experiences with STEM professionals such as scientists, educators, and graduate students to build social networks; (3) furnishing opportunities for participants to develop shared science identities with like-minded individuals; and (4) offering exposure to and preparation for a variety of STEM majors and STEM careers so that youth can engage in discovering possible selves. These findings support our central thesis that long-term engagement in ISE OST programs fosters persistence in STEM.