Purpose: This study investigates the antecedents of career initiative, a proactive behavior, whereby individuals engage in activities to promote their career development. The authors first argue that organizational tenure – the length of time employed within a specific organization – will exhibit a curvilinear or inverted-U-shaped relationship with career initiative. In the early years of an employment relationship, career initiative gradually increases as employees overcome the initial challenges of joining a new organization. However, career initiative will plateau and eventually decline as employees struggle to envision further development. Design/methodology/approach: This study uses a survey design with data collected from the North American operations of a large global telecommunications company. Findings: This study identifies two key mechanisms, both concerning relational context, that drive the curvilinear relationship between organizational tenure and career initiative: mentoring and barriers to networking. Specifically, increased mentoring and reduced barriers to networking both significantly weaken the curvilinear effect. Research limitations/implications: The results suggest that organizations can promote proactive behaviors through employee mentoring and by removing network barriers, particularly for those most at risk for reduced career initiative: early- and especially later-tenure employees. Originality/value: Career initiative is a valued behavior among employees, but individual-level phenomena can be fostered, or inhibited, by relational context. So, while some scholars have found a trend toward “boundaryless” careers, this study reveals the importance of considering how the boundaries and social context within organizations can create an environment in which employee proactivity can flourish.