Even though media literacy education has grown in the United States and around the world, policymakers still lack knowledge about the scope and depth of implementation of media literacy learning activities in the elementary and secondary grades. This study reports how the Media Literacy Implementation Index (MLI) was developed and validated using a quota sample of stakeholders from across all school districts and local communities in Rhode Island. Using an established theoretical framework for media literacy, we first constructed a scale to examine the likelihood that learners may encounter a set of media literacy learning activities. Multiple methods were then used to assess reliability and validity of the instrument including cognitive pretesting, think aloud protocols, a content validation of MLI learning activities in relation to national education standards documents, and statistical validation including principal components analysis to assess internal consistency, reliability, and validity. An easy-to-implement measure of the scope and depth of media literacy education in elementary and secondary schools will be useful to diverse stakeholders including educators, school leaders, librarians, parents, community members, and elected public officials.