OBJECTIVES: We examined the impact of community mobilization (CM) on the empowerment, risk behaviors, and prevalence of HIV and sexually transmitted infection in female sex workers (FSWs) in Karnataka, India. METHODS: We conducted behavioral-biological surveys in 2008 and 2011 in 4 districts of Karnataka, India. We defined exposure to CM as low, medium (attended nongovernmental organization meeting or drop-in centre), or high (member of collective or peer group). We used regression analyses to explore whether exposure to CM was associated with the preceding outcomes. Pathway analyses explored the degree to which effects could be attributable to CM. RESULTS: By the final survey, FSWs with high CM exposure were more likely to have been tested for HIV (adjusted odd ratio [AOR] = 25.13; 95% confidence interval [CI] = 13.07, 48.34) and to have used a condom at last sex with occasional clients (AOR = 4.74; 95% CI =  2.17, 10.37), repeat clients (AOR = 4.29; 95% CI = 2.24, 8.20), and regular partners (AOR = 2.80; 95% CI = 1.43, 5.45) than FSWs with low CM exposure. They were also less likely to be infected with gonorrhea or chlamydia (AOR = 0.53; 95% CI = 0.31, 0.87). Pathway analyses suggested CM acted above and beyond peer education; reduction in gonorrhea or chlamydia was attributable to CM. CONCLUSIONS: CM is a central part of HIV prevention programming among FSWs, empowering them to better negotiate condom use and access services, as well as address other concerns in their lives.