BACKGROUND: The COVID-19 pandemic has resulted in unprecedented disruptions, restrictions, and concerns about physical and mental health. Emerging adulthood, including the first year of college, is associated with declines in healthy eating and physical activity, as well as possible heightened distress. The impact of COVID-19 may exacerbate these concerns. PURPOSE: The purpose of this study was to examine changes in health behaviors and perceived stress in emerging adults over the first year of college and to determine whether prepandemic health behaviors were protective for mental health and stress during the initial changes after the COVID-19 pandemic. METHODS: First-year college students (N = 234, 58.6% female) completed three surveys during their first year of school, the third being after the onset of COVID-19 and during a stay-at-home order. At Time 3, we also assessed symptoms of anxiety and depression. RESULTS: Using linear mixed modeling, sedentary time increased and physical activity decreased over time, but 20%-35% of students reported improvements in these behaviors. Dietary changes appeared mixed, with some improvements noted early during COVID-19. Perceived stress increased over time. Multiple regression indicated that of the health behaviors examined for protective effects on mental health and stress during the pandemic, only diet quality emerged as a significant predictor. CONCLUSIONS: Although notable declines in some health habits were observed over time, including following COVID-19 disruptions, some students reported improved health behaviors. Efforts should be directed at identifying and intervening with students most at risk for poor functioning.