Purpose. The COVID-19 pandemic has imposed unprecedented stressors on individuals globally, leading to significant mental health challenges. This study examines the relationship between perceived adversities experienced during the pandemic-such as stress, risk perception, and loneliness-and the mental health outcomes of individuals with a specific focus on those already grappling with mental disorders. Methods. The study analyzed data from 8,259 adults who participated in surveys between waves 4 (April 2020) and 29 (June 2021) of the Understanding American Study. Participants self-reported their psychiatric diagnosis history and completed self-report measures of mental distress (PHQ-4), perceived stress, risk perception, and loneliness. Results . Those with preexisting comorbid mental disorders reported higher levels of mental distress, COVID-19 risk perception, perceived stress as well as more days experiencing loneliness during the pandemic. Perceived adversities-stress, risk perception, and loneliness-were all positively associated with increased mental distress, indicating the risk factors for mental distress during the pandemic. Among these factors, loneliness was identified as the primary contributor, accounting for 30%-40% of the mental health gap between individuals with and without preexisting mental diagnoses. Also, the detrimental impact of these adversities was more pronounced for those with preexisting mental disorders. Conclusion. The research highlights that those with preexisting mental disorders, particularly those with comorbidities, face an increased risk of experiencing mental distress during the COVID-19 pandemic. These findings underscore the critical importance of providing targeted support and interventions specifically designed for this vulnerable population, with a particular focus on addressing loneliness.