BACKGROUND: Most research on community empowerment provides evidence on engaging communities for health promotion purposes rather than attempts to create empowering conditions. This study addresses this gap. INTERVENTION: Big Local started in 2010 with £271M from the National Lottery. Ending in 2026, it gives 150 relatively disadvantaged1 communities in England control over £1M to improve their neighbourhoods. OBJECTIVE: To investigate health and social outcomes, at the population level and among engaged residents, of the community engagement approach adopted in a place-based empowerment initiative. STUDY DESIGN, DATA SOURCES AND OUTCOME VARIABLES: This study reports on the third wave of a longitudinal mixed-methods evaluation. Work package 1 used a difference-in-differences design to investigate the impact of Big Local on population outcomes in all 150 Big Local areas compared to matched comparator areas using secondary data. The primary outcome was anxiety; secondary outcomes included a population mental health measure and crime in the neighbourhood. Work package 2 assessed active engagement in Big Local using cross-sectional data and nested cohort data from a biannual survey of Big Local partnership members. The primary outcome was mental well-being and the secondary outcome was self-rated health. Work package 3 conducted qualitative research in 14 Big Local neighbourhoods and nationally to understand pathways to impact. Work package 4 undertook a cost-benefit analysis using the life satisfaction approach to value the benefits of Big Local, which used the work package 1 estimate of Big Local impact on life satisfaction RESULTS: At a population level, the impacts on ‘reporting high anxiety’ (–0.8 percentage points, 95% confidence interval –2.4 to 0.7) and secondary outcomes were not statistically significant, except burglary (–0.054 change in z-score, 95% confidence interval –0.100 to –0.009). There was some effect on reduced anxiety after 2017. Areas progressing fastest had a statistically significant reduction in population mental health measure (–0.053 change in z-score, 95% confidence interval –0.103 to –0.002). Mixed results were found among engaged residents, including a significant increase in mental well-being in Big Local residents in the nested cohort in 2018, but not by 2020; this is likely to be COVID-19. More highly educated residents, and males, were more likely to report a significant improvement in mental well-being. Qualitative accounts of positive impacts on mental well-being are often related to improved social connectivity and physical/material environments. Qualitative data revealed increasing capabilities for residents’ collective control. Some negative impacts were reported, with local factors sometimes undermining residents’ ability to exercise collective control. Finally, on the most conservative estimate, the cost–benefit calculations generate a net benefit estimate of £64M. MAIN LIMITATIONS: COVID-19 impacted fieldwork and interpretation of survey data. There was a short 4-year follow-up (2016/20), no comparators in work package 2 and a lack of power to look at variations across areas. CONCLUSIONS: Our findings suggest the need for investment to support community organisations to emerge from and work with communities. Residents should lead the prioritisation of issues and design of solutions but not necessarily lead action; rather, agencies should work as equal partners with communities to deliver change.