As engagement with science, technology, engineering, and mathematics (STEM) increases in after-school programs (ASPs), it is important to examine the impact of this engagement on students' academic achievement, STEM participation, and affinity toward STEM. Results of these examinations can offer insights into both best practices that could be replicated and possible poor practices that could be avoided in ASP sites. This study describes the validation process that was undertaken on an instrument developed to measure science-related attitudes, and education and career trajectories of students participating in a STEM-focused ASP. We then use the validated instrument to draw certain conclusions about the impact of the ASP program on the participants. We propose a model for predicting students' notions about the importance of science for their future and a model for predicting students' enactment of science agency. The study and the derived instrument may be useful for those interested in examining the impact of STEM-focused ASPs on students' attitudes and proclivities toward science.