BackgroundLong COVID symptoms (i.e., experiencing symptoms of COVID‐19 for 3 months post‐COVID‐19) affect individuals’ health and their quality of life (QoL). However, the pathways through which it does so are not fully known.AimThe present study examined the mediating roles of sleep quality, psychological distress, and self‐stigma in the associations between long COVID symptoms and QoL among individuals with mental illness.MethodIndividuals with mental illness (n = 333) were recruited from a psychiatric center in southern Taiwan to participate in the study. Data were collected regarding sleep quality, psychological distress, self‐stigma, and QoL. Independent t‐tests, Pearson correlations, and regression with Hayes’ Process Macro were used to compare groups, examine relationships, and parallel mediation models, respectively.ResultsParticipants with long COVID symptoms had significantly worse sleep quality, psychological distress, physical QoL, and psychological QoL compared to those without symptoms. There were significant relationships between sleep quality, psychological distress, self‐stigma, and QoL. Sleep quality significantly mediated the associations between long COVID symptoms and physical and social QoL. Psychological distress significantly mediated the associations between long COVID symptoms and all domains of QoL, but not self‐stigma.ConclusionThere are alternative pathways (e.g., sleep quality and psychological distress) through which long COVID symptoms may affect the QoL of individuals with mental illness. The findings suggest that individuals with long COVID symptoms have a higher chance of having poor QoL. Therefore, there may be the need for counseling and possible therapy for those who contract COVID‐19, especially among individuals who already have mental illness.