Background: Authentic research experiences and mentoring have positive impacts on fostering STEM engagement among youth from backgrounds underrepresented in STEM. Programs applying an experiential learning approach often incorporate one or both of these elements, however, there is little research on how these factors impact youth's STEM engagement during the high school to college transition. Purpose: Using a longitudinal design, this study explored the impact of a hands-on field research experience and mentoring as unique factors impacting STEM-related outcomes among underrepresented youth. We focus on the high school to college transition, a period that can present new barriers to STEM persistence. Methodology/Approach: We surveyed 189 youth before and up to 3 years after participation in a 7-week intensive summer intervention. Findings/Conclusions: Authentic research experiences was related to increased youths' science interest and pursuit of STEM majors, even after their transition to college. Mentorship had a more indirect impact on STEM academic intentions; where positive mentorship experiences was related to youths' reports of social connection. Implications: Programs designed for continuing STEM engagement of underrepresented youth would benefit from incorporating experiential learning approaches focused on authentic research experiences.