This study tests whether the employer mandate under the Affordable Care Act (ACA) increased involuntary part-time (IPT) employment. Using data from the Current Population Survey between 1994 and 2015, the authors find that IPT employment in 2015 exceeded predictions based on economic conditions and the structure of the labor market. Of greater importance, using difference-in-difference methods, they find that the increase in the probability of IPT employment since passage of the ACA was greater in occupations with a larger share of workers affected by the mandate. The authors’ estimates suggest that approximately 700,000 additional workers without a college degree are in IPT employment as a result of the ACA employer mandate.