Volunteering is known to be associated with well-being among older adults. However, less is known about the psychosocial pathways (e.g., personal mastery, social support) through which this occurs, with past studies tending to rely on cross-sectional data, which are susceptible to selection biases. This study, using longitudinal data, investigates how formal and informal volunteering may affect older adults' quality of life through personal mastery, perceived social support, and received social support. Data are from 2 waves of a nationally representative study of older adults aged 60 years and older in Singapore, conducted between 2016 and 2019 (N = 2,887). We estimate indirect effects using a 2-wave mediation model, relying on bootstrapped confidence intervals for significance testing. We find indirect effects from volunteering to quality of life through perceived social support and personal mastery, but not through received social support. While any type (formal/informal) and frequency (regular/nonregular) of volunteering promotes quality of life through perceived social support, indirect effects through personal mastery are limited to regular volunteering in formal settings. Results provide longitudinal evidence for perceived social support as a key pathway from volunteering to quality of life. Volunteering may be an effective way to improve quality of life by helping older adults feel more supported, even if it may not affect the actual help that they receive. Further, a structured and sustainable environment may be required for volunteering to promote personal mastery (and through it, quality of life) among older volunteers.