Science opportunities in out-of-school time (OST) programs hold potential for expanding access to science, engineering, and technology (SET) pathways for populations that have not participated in these fields at equitable rates (Coalition for Science After School, 2014). This mixed-methods study examines the relationship between the diversity of youth participants and the organizational and program design features of a broad sample of SET-focused OST programs in the USA. Overall, many programs in our study appeared to deliver high-quality programming by providing immersive experiences for youth that included inquiry-based learning and positive youth development. Encouragingly, many programs served large numbers of underrepresented minority youth and girls and these programs often showed the most numerous indicators of high-quality learning experiences. While location and a diversity-oriented organizational mission were related to youth diversity, highly diverse programs enacted their mission by developing partnerships, engaging communities, local leaders, and families, and delivering long-term, supportive programs for youth. Thus, SET-focused OST programs hold great promise in promoting broad access to rich science experiences, yet specific programmatic and organizational features are highly related to the diversity of youth participants.