Previous studies have already linked individual differences to job performance and health outcomes separately, yet researchers are still to investigate parsimonious configurations for the associations of these outcomes concomitantly. Furthermore, variables which mediate these relationships have rarely been investigated. Our study proposes a model of personality characteristics (i.e., proactive personality, core self-evaluation, and psychological capital) which can directly predict work engagement, and indirectly, employees' job performance and their mental health. We collected data from a sample of Romanian workers (N = 365) and tested our model via structural equation modeling. The results provide partial support for our hypotheses. Personality characteristics show an association to work engagement, which in turn predicts employees' performance and mental health. However, core self-evaluations and psychological capital also showed a direct link to the measured outcomes. Our findings provide support for current organizational practices, where fixed traits are used as selection criteria, while malleable states are then developed through interventions to improve employee's performance and health.