Using the Finnish Labour Force Surveys merged with register-based follow-up data, we analyzed how different characteristics of part-time work predict employees’ and entrepreneurs’ employment trajectories in an 8-year follow-up. We analyzed careers by the length of weekly working hours and the reason for part-time work, that is, childcare, studies, health, part-time pension, other voluntary choice, or if full-time work was not available (involuntary part-time). We applied sequence analysis to define work career clusters based on the continuum of spells spent in different labor market statuses, that is, in upper and lower white-collar, manual, or entrepreneurial employment, unemployment, studying, pensioned, or inactivity. According to the results, involuntary part-timers have a significantly higher probability of entering the unemployment trajectory than full-time workers. Those working part-time hours due to care responsibilities were also more likely to face frequent periods of unemployment, whereas part-time work combined with studies was associated with stable white-collar trajectories. Our results also show that weakened labor outcomes following marginal part-time jobs associate with disability retirement instead of unemployment later in time, most probably determined by ill health. Therefore, we suggest further studies to consider marginal part-time workers’ health as the determinant of weakening career outcomes. Overall, our results highlight the need to improve part-time working conditions, a concern that organizations like the OECD have also raised. This improvement could reduce the risk of unemployment, promote health, extend work careers, and consequently increase the employment rate.