Co-curricular engagement in postsecondary education provides students with a breadth of opportunity for leadership-focused developmental experiences. However, few studies have qualitatively examined in detail how alumni describe years later how formal co-curricular involvement contributes to their development. Such lack of attention has resulted in what we describe as a "hidden leadership curriculum" embedded in co-curricular engagement. To address this gap in the literature, we explored the leadership experiences of 25 recent alumni who were engaged within various co-curricular organizations, and the leadership lessons relevant to their professional success that they report learning as a direct result of their involvement. Findings from this study reveal how the inherent organizational challenges embedded within co-curricular engagements lead students to develop a more interdependent, relational conception of effective leadership behaviors. These conceptualizations place value on collective group engagement and decision-making, and help students recognize group needs over individual desires. In addition, many participants reported acquiring the value of generativity -- building a leadership pipeline -- within their respective co-curricular organizations. We discuss practical implications, such as the central role of challenging experiences within the co-curriculum, and suggest future research recommendations.