BACKGROUND: In England, during 2009-2014 the 'Time to Change' anti-stigma programme has included a social marketing campaign (SMC) using mass media channels, social media and social contact events but the efficacy of such approach has not been evaluated yet. METHODS: The target population included people aged between mid-twenties/mid-forties, from middle-income groups. Participants were recruited through an online market research panel, before and after each burst of the campaign (with a mean number of unique participants per each burst: 956.9±170.2). Participants completed an online questionnaire evaluating knowledge [Mental Health Knowledge Schedule (MAKS)]; attitudes [Community Attitudes toward Mental Illness (CAMI)]; and behaviours [Reported and Intended Behaviour Scale (RIBS)]. Socio-demographic data and level of awareness of the SMC were also collected. RESULTS: A total of 10,526 people were interviewed. An increasing usage of the SMC-media channels as well as of the level of awareness of SMC was found (P<0.001). Being aware of the SMC was found to be associated with higher score at MAKS (OR=0.95, CI=0.68 to 1.21; P<0.001), at 'tolerance and support' CAMI subscale (OR=0.12, CI=0.09 to 0.16; P<0.001), and at RIBS (OR=0.71, CI=0.51 to 0.92; P<0.001), controlling for confounders. CONCLUSION: The SMC represents an important way to effectively reduce stigma. Taking into account these positive findings, further population-based campaigns using social media may represent an effective strategy to challenge stigma.