We explore the understudied role of program staff in an out-of-school time (OST) program at a large science museum, which may be especially relevant for supporting underrepresented minority (URM) youth's interest in science, technology, engineering, or math (STEM) careers. Using a sequential explanatory mixed-method design, we surveyed 167 program alumni on their science attitudes, career interests, and memories about how the program compared to experiences at home, school, and with friends. We followed that with 49 interviews with alumni. Findings show that, while in the program, alumni who identify as women reported a much greater increase in their STEM career interest than those who identify as men. Interviews suggest this may be related to different types of staff relationships between the genders. We interpret results through the lens of positive youth development and offer recommendations for OST program providers and researchers.