Background: Co-curricular student outcomes research has focused on connecting outcomes to activities based on the co-curricular type. Less work has explored what aspects of those co-curricular activities could lead to student outcomes. Purpose: Our research aimed to identify common elements of co-curricular activities that connected to students’ development of professional, career, or personal outcomes and can inform how we study and design co-curricular activities in engineering. Design: We recruited participants from one biomedical engineering (BME) program. We used a one-year series of four semi-structured interviews with fourteen upper-level BME students to explore students’ perceptions of their co-curricular learning. Using a qualitative, causal analysis approach, we identified elements of students’ co-curricular experiences in research or a multi-disciplinary design team, as well as other co-curricular experiences (e.g., internships, professional societies), that linked to professional, career, or personal learning outcomes that have been previously identified as important in engineering education. Findings: We identified patterns of connections between unique “experience elements” and a variety of “outcome categories” through participant activities we called “participant actions.” The most prevalent connections—those experience elements and participant actions that connected to multiple outcome categories—included the experience elements Independent Project Work, Project Work That Engages Multiple Disciplines, STEM Education Opportunities, and Mentorship from a Skilled Other as well as a participant action Reflecting on Experience. We found connections to the outcome categories of Leadership, Design, Business, Interdisciplinary Competence, Disciplinary Competence, Communication, and Career Direction Outcomes. Conclusions: Based on our findings, educators and mentors should consider the value of supporting students’ decision-making autonomy and multidisciplinary interactions in projects to support learning. They could also incorporate opportunities for students to teach each other technical content, receive structured mentorship, and reflect on their experiences as they are happening. Further, this work demonstrates a need to explore co-curricular learning processes in new ways that can lead to better understandings of students’ learning processes.