Background <p>Mental health represents a growing public health challenge, exacerbated by recent global and regional crises. This study aimed to identify sociodemographic, lifestyle, work- and health-related correlates of key mental health outcomes - primarily depression, insomnia, and job burnout - in a large sample of adult internet users in Poland.</p> Methods <p>We analyzed pooled cross-sectional data from 1 196 102 adults participating in an annual web-based health survey in Poland between 2020 and 2024. Ordinal and binary logistic regression models (2021–2024 waves; <i>N</i> = 655 427) were used to assess associations between predictors (sociodemographics, lifestyle factors, chronic diseases, stress, social relationships, work characteristics) and self-reported mental health outcomes. Self-rated general health, neurological diseases, memory complaints, and frequent headaches were also assessed as secondary outcomes.</p> Results <p>Significant gender disparities were observed, with women reporting higher odds of depression, insomnia, and job burnout (<i>p</i> &lt; 0.001). Daily stress and infrequent social contact were the strongest correlates, markedly associated with increased odds of all adverse mental health outcomes (<i>p</i> &lt; 0.001). The presence of chronic diseases (particularly arthritis and heart disease) and specific lifestyle factors (e.g., lack of physical activity and unhealthy dietary patterns) were also significantly associated factors. Conversely, regular physical activity and frequent social contact were protective.</p> Conclusions <p>Mental health outcomes among Polish adult internet users are strongly associated with a complex interplay of psychosocial, health, and lifestyle factors. High stress, poor social contact, and chronic illness burden are major correlates of poor mental health. These findings emphasize the need for targeted, broad-based public health interventions promoting stress management and social connection.</p>