Objectives. To understand the processes involved in effective social marketing of mental health treatment. Methods. California adults experiencing symptoms of probable mental illness were surveyed in 2014 and 2016 during a major stigma reduction campaign (n = 1954). Cross-sectional associations of campaign exposure with stigma, treatment overall, and 2 stages of treatment seeking (perceiving a need for treatment and use conditional on perceiving a need) were examined in covariate-adjusted multivariable regression models. Results. Campaign exposure predicted treatment use overall (odds ratio [OR] = 1.82; 95% confidence interval [CI] = 1.17, 2.83). Exposure was associated with perceived need for services (OR = 1.64; 95% CI = 1.09, 2.47) but was not significantly associated with treatment use in models conditioned on perceiving a need (OR = 1.52; 95% CI = 0.78, 2.96). Exposure was associated with less stigma, but adjustment for stigma did not affect associations between exposure and either perceived need or treatment use. Conclusions. The California campaign appears to have increased service use by leading more individuals to interpret symptoms of distress as indicating a need for treatment. Social marketing has potential for addressing underuse of mental health services and may benefit from an increased focus on perceived need.