Case management programs assisting patients with social needs may improve health and avoid unnecessary health care use, but little is known about their effectiveness. This large-scale study assessed the population-level impact of a case management program designed to address patients’ social needs. We used a single-site randomized encouragement design with administrative enrollment from an eligible population and intention-to-treat analysis. Study participants were enrolled between August 2017 and December 2018 and followed for 1 year. (ClinicalTrials.gov: NCT04000074). Participants were located in Contra Costa County, an economically and culturally diverse community in the San Francisco Bay Area and the study had a total of 57,972 randomized enrollments of adult Medicaid patients at elevated risk for health care use (top 15%) to the intervention or control group. Enrollees were offered 12 months of social needs case management, which provided more intensive services to patients with higher demonstrated needs. Medical use was measured via emergency department (ED) visits and inpatient admissions, some of which were classified as avoidable. Participants in the intervention group visited the ED at ratios of 0.96 (95% CI, 0.91 to 1.00) for all visits and 0.97 (CI, 0.92 to 1.03) for avoidable visits relative to the control group. The intervention group was hospitalized at ratios of 0.89 (CI, 0.81 to 0.98) for all admissions and 0.72 (CI, 0.55 to 0.88) for avoidable admissions. Only 40% of the intervention group engaged with the program. The program was in continual development during the trial period. Although social needs case management programs may reduce health care use, these savings may not cover full program costs. More work is needed to identify ways to increase patient uptake and define characteristics of successful programs.