The objective of this study was to assess the impact of a community-based healthy weight intervention on child weight and fitness. Cambridge Public Schools (CPS) have monitored BMI and fitness annually since 2000. Annual increases of overweight and obesity from 2000 (37.0%) to 2004 (39.1%), triggered a multidisciplinary team of researchers, educators, health care, and public health professionals to mobilize environmental and policy interventions. Guided by the social-ecological model and community-based participatory research (CBPR) principles, the team developed and implemented Healthy Living Cambridge Kids (HLCK), a multicomponent intervention targeting community, school, family, and individuals. The intervention included city policies and community awareness campaigns; physical education (PE) enhancements, food service reforms, farm-to-school-to-home programs; and family outreach and “BMI and fitness reports”. Baseline (2004) to follow-up (2007) evaluation design assessed change in children's weight and fitness status. A cohort of 1,858 K-5th grade children participated: 37.3% black, 14.0% Hispanic, 37.1% white, 10.2% Asian, 1.7% other race; 43.3% were lower income. BMI z-score (0.67–0.63 P < 0.001) and proportion obese (20.2–18.0% P < 0.05) decreased, and mean number of fitness tests (0–5) passed increased (3.7–3.9 P < 0.001). Whereas black and Hispanic children were more likely to be obese at baseline (27.0 and 28.5%, respectively) compared with white (12.6%) and Asian (14.3%) children, obesity among all race/ethnicity groups declined. Concurrent with a 3-year community intervention, modest improvements in obesity and fitness were observed among CPS children from baseline to follow-up. The CBPR approach facilitated sustaining policies and program elements postintervention in this diverse community.