Even though person-job fit (PJ fit) is a crucial predictor of employees' overall engagement and performance in their jobs, few studies have identified the mechanisms that enhance PJ fit during the employment relationship. Further, the models that do predict how PJ fit evolves over time are predominantly based on the idea that fit improves through individual adjustment processes by workers. This paper provides a new lens on PJ fit that is based on strengths theory, proposing that strengths-based leaders play a critical role in enhancing both dimensions of PJ fit, i.e. needs-supplies (NS) and demands-abilities (DA) fit, by encouraging employees to use their unique strengths. Furthermore, based on theorizing on substitutes for leadership, proactive career management, and situational strength theory we test the idea that high levels of proactive personality and job autonomy may partly compensate for a lack of strengths-based leadership. We collected three waves of data with two-month time lags from a representative sample of 308 Dutch workers, resulting in 906 datapoints. Results of multi-level path modeling indicate that strengths-based leadership is indeed positively related to both DA fit and NS fit and mediated by strengths use at the within-person level. Further, our results indicate that the combination of high job autonomy and high proactive personality partly compensates for the absence of strengths-based leadership on the within-person level. We conclude that strengths-based leadership is particularly important to facilitate strengths use and PJ fit of employees who have a low propensity to be proactive and/or do not have a high degree of autonomy in their job. We discuss the implications of these findings for research and practice.