In the last decade, organizational researchers have increasingly recognized the value of studying personality states at work. This line of research has to date mainly focused on outcomes of and situational antecedents of personality state change at work. In this study, we draw on social cognitive theory of self-regulation and the social investment principle to test if a structured intervention can bring about changes in employees’ personality states at work. Specifically, we investigate the effect of a four-week low-dose mindfulness intervention delivered via a mobile application on employees’ personality state change in a multi-group experiment with a passive and an active control group. Employees (N¼162), either practicing mindfulness, doing brain training exercises, or not receiving any treatment were tracked in a diary study across a period of four weeks. Results of growth curve analyses showed that compared to the control groups, the mindfulness intervention led to significantly more increases in employees’ daily mindfulness, emotional stability and agreeableness across the four-week period. Furthermore, daily agreeableness and emotional stability mediated the effect of the intervention on daily job satisfaction and performance. No intervention effects were found for daily openness, conscientiousness, and extraversion. Implications are discussed for research and practice.