This article examines the relationships between professionals’ early career developmental networks and their knowledge creation. An empirical study of 167 novice professionals finds that novices’ knowledge creation benefits from both closed and open structures in developmental networks, but the effects depend on whether they work with a prominent mentor. Traditional developmental networks, characterized by dense structures, relate positively to novices’ future knowledge creation, except when there is a prominent mentor in the network, suggesting that density and prominence are substitutable forms of social capital. Entrepreneurial developmental networks, where novices are positioned between disconnected mentors within the developmental network, relate negatively to future knowledge creation for all novices, including those with prominent mentors. Finally, the extent to which the developmental network reaches collaborators in the broader professional network relates positively to novices’ future knowledge creation, and this effect is strongest for those novices with prominent supervisors in the developmental network. Overall, these findings suggest important implications for understanding novices’ developmental networks and their knowledge creation.