Background: Internationally, nutrition standards for school food have been implemented. Food service professionals are key to their implementation. This mixed-methods systematic review provides an overview of the barriers and facilitators faced by food service professionals when implementing food/nutrition standards and provides an important link between policy and practice. Methods: Peer-reviewed and grey literature were searched across electronic databases and public health organization websites. The Consolidated Framework for Implementation Research (CFIR) was used as a framework in the analysis. Findings: Twenty-nine studies met the eligibility criteria. The most frequently cited barriers to the CFIR constructs/subconstructs were linked to the internal school setting. This included staffing, materials and equipment, and funding. Frequently cited facilitators coded to CFIR included external partnerships/connections and staff motivation to implement the standards. Implications for School Health Policy, Practice, and Equity: Our findings highlight not only well-documented challenges such as funding and staffing but also the novel and pivotal insights from food service professionals that point to practical, systems-level solutions. Conclusion: Researchers and practitioners can utilize the results to devise strategies to heighten implementation, as well as capitalize on factors that aid in the implementation process.