The purpose of this study was to examine the longitudinal impact of the COVID-19 pandemic on public-school teachers' mental well-being. The present study surveyed a total of 4,926 public school teachers from across Illinois in the fall of 2019, 2020, and 2021. The survey measured teachers' mental well-being using the Warwick-Edinburg Mental Well-Being Scale. The results found that on average, the mental well-being of public-school teachers significantly declined between the fall of 2019 and the fall of 2021. The present study discusses the implications of these findings on school-based mental and behavioral health interventions for teachers.