Introduction The burden of mental illness is pervasive. Improved methods for early detection of mental ill-health and comprehensive understandings of factors associated with declining mental health are needed. Workplace social support is an important factor for addressing employee’s mental health. However, only a few studies have explored how workplace social support impacts on mental health from a functional perspective.

Objectives To investigate the prevalence of limited Mental Health Functioning (MHF) among Swedish workers and the association between workplace social support and MHF.

Methods This cohort study used a sample of 4979 employees from Stockholm County who responded to population-based questionnaires in 2000 and 2002. Two subsamples were explored: workers without baseline limitations in MHF (n=2844) and workers with baseline limitations in MHF (n=2135). Exposure to workplace social support was estimated using the Swedish Demand-Control-Support Questionnaire. Follow-up data on MHF were taken from the World Health Organisation’s Brief Disability Questionnaire. Multinomial regression analysis generated odds ratios (OR) with 95% confidence intervals (CI95%) adjusting for age, sex, education, socio-economic classification, cohabitation and social support outside the workplace.

Results In total, 43% (37% moderate and 6% severe) of workers had limited MHF at baseline. A higher level of workplace social support was associated with a reduced likelihood of moderate functional limitations (AOR 0.92 95%CI, 0.89–0.95) or severe functional limitations (AOR 0.87 95%CI, 0.80,0.94) two years later. Among workers with pre-existing functional limitations, increased workplace social support was associated with improved MHF at follow-up (AOR 1.08 95%CI, 1.05, 1.12).

Conclusions Limited MHF was prevalent among workers. We observed a protective relationship between increased workplace social support and limited MHF. Increased social support was also associated with improved MHF among workers with limited MHF. Interventions that increase workplace social support might protect against a deterioration in MHF and improve MHF among workers with pre-existing limitations.