Libraries may serve as a way to bridge educational equity gaps through informal learning programs. This is especially true in the United States where inequality is a growing concern at local, state, and federal levels. While the informal learning literature within the United States highlights innovative practitioner reflections and design cases, few articles have attempted to synthesize empirical trends that outline cognitive and affective learning outcomes. To address this gap, this systematic review explored empirical studies of K-12 informal learning programs within United States libraries. The findings suggest a focus on initiatives within public libraries and STEM learning, especially in urban settings. That said, many studies presented qualitative data and reported on affective learning outcomes, which leads to questions as to the extent of learning gains and replicability. Implications for practice are discussed.