Despite the success of informal STEM programs, one ongoing challenge is sustaining learners' interest beyond the programs' initial impact (Morris et al., 2019). Although some longitudinal studies suggest that such programs can have a lasting impact on girls in STEM (e.g., McCreedy & Dierking, 2013), further research is needed to identify the distinctive features of successful programs that create meaningful and enduring change. The present study seeks to address this gap with a retrospective exploration of the experiences of the first cohort of participants in a longstanding girls' STEM program, Girls Excelling in Math and Science (GEMS). In interviews and focus groups, women now in their 40s described their experiences in the program 30 years ago. The study is guided by the research question: "What distinctive features of GEMS as an informal learning environment emerge from the retrospective narratives of the original participants?" Guided by sociocultural theory, the author used thematic analysis to unpack participants' narratives to reveal three key features of the program. The study provides actionable insights for designing effective afterschool programs and informal learning opportunities to engage girls and learners from historically underrepresented communities in STEM fields.