With the growing interest in the joint effects of individual and contextual factors in predicting team member proactivity, this paper examines why and when pursuing one's career calling can lead to team member proactivity. Drawing on the Work as a Calling Theory, we propose that “living out a calling” explains why employees' perceived career calling positively relates to team member proactivity and especially when the employee receives high levels of mentoring support. Our hypotheses are tested using a multisource and time-lagged study design with a sample of 296 dyads of Chinese employees and their direct supervisors. We found support for the mediating role of living out a calling (Time 2) in the positive relationship between perceiving a calling (Time 1) and team member proactivity (Time 3). Mentoring (Time 2) moderated the perceiving a calling and living out a calling link such that when employees received more mentoring, the relationship was positive, whereas under lower levels of mentoring, the relationship was negative. Similarly, the indirect relationship between perceiving a calling and team member proactivity through living out a calling was positive at higher levels of mentoring, but the relationship was negative at lower levels of mentoring.