Out-of-school time programs focused on science, technology, engineering and mathematics (STEM) have proliferated recently because they are seen as having potential to appeal to youth and enhance STEM interest. Although such programs are not mandatory, youth are not always involved in making the choice about their participation and it is unclear whether youth’s involvement in the choice to attend impacts their program experiences. Using data collected from experience sampling, traditional surveys, and video recordings, we explore relationships among youth’s choice to attend out-of-school time programs (measured through a pre-survey) and their experience of affect (i.e., youth experience sampling ratings of happiness and excitement) and engagement (i.e., youth experience sampling ratings of concentration and effort) during program activities. Data were collected from a racially and ethnically diverse sample of 10–16 year old youth (n = 203; 50% female) enrolled in nine different summer STEM programs targeting underserved youth. Multilevel analysis indicated that choice and affect are independently and positively associated with momentary engagement. Though choice to enroll was a significant predictor of momentary engagement, positive affective experiences during the program may compensate for any decrements to engagement associated with lack of choice. Together, these findings have implications for researchers, parents, and educators and administrators of out-of-school time programming.