This article assesses the effects of formal mentoring on workplace networks. It also provides conceptual clarity and empirical evidence on expected gender differences in the effects of such programs. Qualitative interviews with 40 past participants in a formal mentoring program at a software laboratory in Beijing, China, provide insight into the core mechanisms by which such programs produce network change: access to organizational elites, participation in semiformal foci, enhanced social skills, and legitimacy-enhancing signals. These mechanisms are theorized to lead to an expansion in proteges' networks, relative to those of non-participants in formal mentoring. Legitimacy-enhancing signals are theorized to enable female proteges to derive greater network benefit from formal mentoring than their male counterparts. Empirical support for these propositions comes from a longitudinal quasi-experiment involving 75 employees who experienced the treatment of formal mentoring and 64 employees in a matched control group. A second empirical strategy, which exploits exogenous variation in the timing of treatment and enables a comparison of the post-program networks of one treated group to the pre-program networks of another treated group, provides corroborating support. These findings contribute to research on the efficacy of formal mentoring, gender and workplace networks, and the cumulative advantage or disadvantage that can arise from network change.