Programming aimed at promoting positive student mental health and reducing or preventing mental-ill health is common within schools in Ontario. A brief, valid, and scalable measure was needed to assess the organizational conditions, or the capacity, readiness, and resources of schools to successfully implement and sustain this programming. In partnership with School Mental Health Ontario, an intermediary organization that facilitates uptake of student mental health programming in schools across the province, the objectives of the current study were to adapt and evaluate the psychometric properties of the School Organizational Conditions for Mental Health Programming Measure for principals. An 18-item measure was completed by 623 principals across the province (from 35 school boards (i.e., districts); 79% elementary, 16% secondary, and 5% both elementary and secondary). A measure of mental health emergency readiness was used to assess convergent validity. Results from item response theory and exploratory and confirmatory factor analysis supported a final 14-item measure assessing four domains: (1) School Mental Health Leadership, (2) Engagement and Collaboration with External Partners, (3) Mental Health Strategy and Action Planning, and (4) Data-Informed Quality Improvement. The final measure demonstrated excellent internal consistency ([alpha]s = 0.83-0.91) and measurement invariance across elementary and secondary schools, and schools in urban and rural areas. Scores were positively associated but not redundant with mental health emergency readiness (rs = 0.31-0.42). The School Organizational Conditions for Mental Health Programming Measure is a brief measure that shows promising psychometric evidence for evaluating the organizational conditions of schools for supporting student mental health-related programming.