In response to unwaveringly high attrition from STEM pathways, STEM Intervention Programs (SIPs) support STEM students in effort to increase retention. Using mixed methods (survey and focus groups), we studied students at one university who were either supported or unsupported by SIPs to understand how students may differ in experiences believed to contribute to STEM persistence. We evaluated: sense of belonging, scientific self-efficacy, scientific community values, scientific identity, and STEM involvement. The enrollment status of students two and a half years postsurvey was also tracked. SIP students reported significantly higher science identity and sense of belonging and were more involved in STEM-related activities than counterparts unsupported by SIPs. Differences in these measures were correlated with race/ethnicity, college generation status, and age. Notably, SIP students had higher odds of persisting in STEM than students not supported by SIPs. Focus group data provide additional meaning to the measured survey constructs and revealed nuanced qualitative differences between SIP and non-SIP student experiences. Overall, being involved in a SIP at our institution trends positively with theoretical models that explain STEM student persistence. SIPs have the potential to provide and/or facilitate meaningful and critical support, and students without those intentional supports may be left behind.