Despite increasing importance of proactive employees, who take initiatives to create change that benefit organizations, the understanding of the underlying motivational mechanisms behind how distal proactivity predisposition unfolds and results in the enactment of proactive work behaviors is less well-understood. Examining the interaction of individual characteristic and situation provides better understanding of the motivational mechanisms behind how employee proactivity unfolds. Specifically, I drew from motivated information processing model to propose prosocial motivation as an important mediating mechanism, which ultimately determines proactive work behaviors. I further identified transformational leadership as an important situational factor that can promote individual proactivity among employees with low FRO. Using three-wave source data from 114 employees, the key findings are (1) FRO is positively related to proactive work performance through mediation of prosocial motivation; (2) consistent with trait activation theory, in the absence of transformational leadership, FRO had stronger indirect effect on proactivity via prosocial motivation. This implies that for employees with low FRO, transformational leaders can have a greater role to play in enhancing prosocial motivation, leading to subsequent proactive work behaviors. Hence, the findings underscore the importance of the interplay of person and situation in shaping individual’s motivation to engage in proactive work behaviors.