Internships are widely promoted extra-curricular experiences that can have positive impacts on student outcomes, yet how specific elements of internships contribute to these outcomes and facilitate learning is understudied. In this sequential mixed-methods study, we use sociocultural learning theory to interpret data from surveys (n = 435) and focus groups (n = 52) with students at five postsecondary institutions. After stepwise linear regression analyses indicated that supervisor behaviors were significantly associated with intern satisfaction and career development, analyses of qualitative data revealed features of positive (clear communication, availability, feedback) and negative (unavailability, inattention to learning) aspects of supervision. These results highlight the value of legitimate peripheral participation in internships, and the need for colleges and employers to carefully design and monitor these pedagogic spaces.