The beneficial potential of flow experience is highlighted by research demonstrating positive associations between flow and wellbeing. Flow has also been associated with stress—a relationship that has not received much attention in the context of work. Unfinished tasks have been identified as a crucial work-related stressor in recent occupational stress research. Extending previous research, we examine in two consecutive studies how unfinished tasks are related to flow and whether flow plays a mediational role between unfinished tasks and wellbeing. Study 1 adopted a cross-sectional design, with 93 employees taking part in an online survey assessing their work experiences during the previous two weeks. Study 2 employed a short-term diary design and 149 participants (85 employees and 64 students) responded to our survey at three points of measurement: after work/study, before going to bed, and in the next morning. Results from both studies provided evidence for a negative quadratic relationship between unfinished tasks and flow at work/study, with low to medium levels of unfinished tasks being unrelated to flow, while high levels of unfinished tasks were negatively associated with flow. The relationship of unfinished tasks at work/study and flow during an evening activity was negative. Both studies supported the postulated mediating role of flow in the relationship between unfinished tasks and wellbeing. Thus, finishing tasks during the day and in particular before leaving the workplace is a helpful condition to experiencing flow both at work and during non-work activities and to fostering wellbeing.