The experiences students have in and out of school can influence the way they think about STEM and the career decisions they make. The purpose of this mixed-methods study was to determine which learning experiences students perceived as meaningful to the development of their STEM self-efficacy and career choices, and how those perceptions differed between STEM and non-STEM students. A survey of 312 undergraduates elicited these perceptions while a follow-up interview with eight participants from the sample expanded on the nuances of their decision-making. Results from the study suggested that students enrolled in a STEM major were more likely to have had career-specific experiences as part of their curriculum, early experiences and family involvement in STEM, participation in STEM extracurriculars, and strong performance in STEM coursework. Interviews revealed how students from both STEM and non-STEM pathways engaged in these experiences and their perceptions of how the experiences influenced their STEM decision-making. Synthesis of quantitative and qualitative results suggested that perceived innate interest may actually be the result of early experiences and family-related activities, and students' experiences with STEM extracurriculars and career-focused activities affected how they viewed their career path. These findings suggest implications for how career-specific curriculum, family support, and access to out-of-school opportunities can benefit students' STEM beliefs and career actions.