This paper seeks to develop a better understanding of job performance in the context of a knowledge management system (KMS) implementation. This work adopts the context theorizing approach that informs the conceptualization of KMS use and identification of contingency factors. Specifically, the literature on rich system use is adapted to develop the construct in the context of a KMS. The literature related to task, system, user, and leadership is also drawn upon to identify four contingency factors—task nonroutineness, perceived support for contextualization, absorptive capacity, and transformational leadership—that affect the KMS use and job performance relationship. The paper argues that rich use of a KMS positively affects job performance and the four contingency factors moderate this relationship. A mixed methods approach that includes a quantitative study (n = 1,441) among knowledge workers in seven business units of a large organization in the finance industry was used to validate the theoretical model. A follow-up qualitative study (n = 48) was conducted in one business unit to cross-validate the findings and explain unsupported findings. Data were collected from multiple sources (i.e., surveys, interviews, and system archives). The results largely supported the model. Theoretical and practical implications of the results are discussed.