Background: Our objective was to examine the association between school wellness committees and implementation of nutrition wellness policies and children's weight status and obesity-related dietary outcomes. Methods: A cross-sectional study was conducted of 4790 children aged 4-15 years recruited from 130 communities in the Healthy Communities Study. Multilevel statistical models assessed associations between school wellness policies and anthropometric (body mass index z-score [BMIz]) and nutrition measures, adjusting for child and community-level covariates. Results: Children had lower BMI z-scores (-0.11, 95% confidence interval [CI]: -0.19, -0.03) and ate breakfast more frequently (0.14 days/week, 95% CI: 0.02-0.25) if attending a school with a wellness committee that met once or more in the past year compared to attending a school with a wellness committee that did not meet/did not exist. Children had lower added sugar (p < 0.0001), lower energy-dense foods (p = 0.0004), lower sugar intake from sugar-sweetened beverages (p = 0.0002), and lower dairy consumption (p = 0.001) if attending a school with similar or stronger implementation of the nutrition components of the school wellness policies compared to other schools in the district. Conclusions: A more active wellness committee was associated with lower BMI z-scores in US schoolchildren. Active school engagement in wellness policy implementation appears to play a positive role in efforts to reduce childhood obesity.