Background: The share of workers who work part-time because full-time jobs are not available remains larger compared to the period prior to the 2008 crisis. For part-time workers, being available to work more hours than offered may have negative mental health implications. Method: Drawing on two nationally representative British surveys, we tested whether working less than 30 hours per week while preferring to work longer hours (underemployment) is associated with increased psychological distress. Distress was assessed using responses to the 12-item General Health Questionnaire in both samples. Results: In the National Child Development Study (N = 6,295), propensity score estimates indicated that the hours-underemployed workers experienced higher levels of psychological distress (β = 0.25, p <0.001) than full-time workers matched on observable characteristics, including prior distress levels. Fixed effects estimates using 18 years of the British Household Panel Survey (N = 8,665) showed that transitioning from full-time employment to underemployment predicted an increase in distress levels (β = 0.19, p <0.01). Conversely, transitioning from underemployment to full-time employment forecasted a reduction in distress (β = -0.18, p <0.001). On average, job earnings and perceptions of job security explained a small (≈ 10%) portion of the potential psychological impact of hours-underemployment. Conclusions: These findings highlight the possibility that underemployment among part-time workers may have detrimental psychological consequences. Policy interventions geared towards improving career opportunities for part-time workers would potentially ameliorate losses in psychological well-being experienced by this group.