Intraorganizational networks are critical for careers, yet Black employees derive fewer benefits from networks than their White counterparts, partly because they form fewer cross-race instrumental ties. Extant research has attributed these disparities to interpersonal mechanisms whereby dissimilar individuals are less likely to connect. This paper moves beyond dyadic explanations by theorizing how triadic closure—whereby shared contacts increase the likelihood of tie formation—varies by race. We argue that the presence of shared contacts creates the opportunity structure for racial disparities in ties formed by reducing transparency and diffusing accountability, particularly when those ties provide access to career-advancing work. Using a longitudinal network dataset combining personnel and staffing records of 2,481 new hires in a professional services firm, we estimate the causal effect of having shared contacts on subsequent year-end tie formation. We find that the effect of shared contacts on subsequent tie formation is approximately 94% lower for Black than for White new hires, although no such gap exists when ties form in the absence of shared contacts. These findings advance our understanding of the network drivers of racial disparities, demonstrate that triadic closure is socially differentiated, and show how networks generate unequal access to career-advancing opportunities.