Multi-sector partnerships are core in efforts to improve population health but are often not as fully developed or positioned to advance health and equity in their communities as believed to be. Therefore, measuring the collaborations multi-sector partnerships undertake is important to document the inputs, processes, and outcomes that evolve as they work together towards achieving their goals, which ultimately creates a greater sense of shared accountability. In this study we present the development and validation of the Assessment for Advancing Community Transformation (AACT), a new tool designed to measure readiness to advance health and health equity. Development of the AACT included initial item pool creation, external evaluation from five subject matter experts, and pilot testing (including user feedback surveys) among 103 individuals. Validation of the AACT was performed using a series of confirmatory factor analyses on an expanded dataset representing 352 individuals from 49 multi-sector collaboratives across the United States. The results of our study indicate the items in the AACT align to six domains created during the scale development process, and that the tool demonstrates desirable measurement characteristics for use in research, evaluation, and practice.