Personality, as a frequently used predictor of job performance, has often been criticized for its low criterion-related validity when compared to cognitive tests and some other predictors. The present study investigated incremental validity of narrow-trait personality to distinguish predictive from non-predictive facets of Conscientiousness and Extraversion dimensions. In addition, some intermediate mechanisms that may link the two personality dimensions with the criterion, such as different types of person-environment (P-E) fit and job involvement variables, were tested as well. The institution's job performance scale, NEO-PI-3 personality scale, person-organization (P-O) fit, needs-supplies (N-S) fit, demands-abilities (D-A) fit, job involvement questionnaire (JIQ), and demographic measures were administered in an online survey to 295 professional and civil service employees of a midsize Midwestern university. The sample was predominantly female and Caucasian with a mean age of 45.8 years and a median length of current employment of 5.1 years. Both personality dimensions were positively related to overall job performance. Conscientiousness was a stronger predictor of task performance, whereas Extraversion was related more consistently to contextual performance. In stepwise multiple regression analyses containing facets of personality dimensions as predictors of overall job performance, Competence emerged as the only facet of Conscientiousness, and Warmth and Assertiveness as the only facets of Extraversion that accounted for a meaningful amount of variance in the criterion. The use of narrow-trait personality to predict overall job performance enhances criterion-related validity of the construct and renders it a more efficient predictor of job performance than global-trait personality. Among the potential mediators, P-O and D-A fit partially mediated the personality-performance relationship providing evidence for the importance of perceptions of congruence in values and the ability to meet demands of the job. Current results are considered in light of limitations. Implications for theory, research, and practice, as well as future research directions are discussed.