Background: This study tested the hypothesis that written district wellness policies are associated with higher rates of implementation of nutrition and physical activity practices. Methods: Written wellness policies and building level practices were assessed for schools (N = 295) within high-need districts (N = 70) in New York State. The relationship between policies and practices was measured using multi-level mixed-effects logistic regressions. Results: Overall, stronger written district policies significantly increase the likelihood of practice implementation in schools. This relationship is strongest for physical education and physical activity items, followed by nutrition standards for competitive foods in middle and high schools. Most elementary schools implemented nutrition practices with or without a policy and there were differences in implementation rates between elementary and middle/high schools. When examined separately, policies were for the most part not significantly associated with implementation of corresponding practices. Conclusions: Strong and comprehensive written policies are associated with higher rates of practice implementation overall, but the consistency of this relationship varies by policy-practice domain. The newer policy topics areas of school wellness promotion and marketing were less frequently included in written policies. Future research should examine whether districts that strengthen their written policies achieve greater implementation over time.