Abstract Background Despite antibiotic stewardship programs existing in most acute care hospitals, there continues to be variation in appropriate antibiotic use. While existing research examines individual prescriber behavior, contextual reasons for variation are poorly understood. Methods We conducted an explanatory, sequential mixed-methods study of a purposeful sample of 7 hospitals with varying discharge antibiotic overuse. For each hospital, we conducted surveys, document analysis, and semi-structured interviews with antibiotic stewardship and clinical stakeholders. Data were analyzed separately and mixed during the interpretation phase, where each hospital was examined as a case, with findings organized across cases using a strengths, weaknesses, opportunities, and threats framework to identify factors accounting for differences in antibiotic overuse across hospitals. Results Surveys included 85 respondents. Interviews included 90 respondents (31 hospitalists, 33 clinical pharmacists, 14 stewardship leaders, 12 hospital leaders). On surveys, clinical pharmacists at hospitals with lower antibiotic overuse were more likely to report feeling: respected by hospitalist colleagues (P = .001), considered valuable team members (P = .001), and comfortable recommending antibiotic changes (P = .02). Based on mixed-methods analysis, hospitals with low antibiotic overuse had 4 distinguishing characteristics: (1) robust knowledge of and access to antibiotic stewardship guidance; (2) high-quality clinical pharmacist–physician relationships; (3) tools and infrastructure to support stewardship; and (4) highly engaged infectious diseases physicians who advocated stewardship principles. Conclusions This mixed-methods study demonstrates the importance of organizational context for high performance in stewardship and suggests that improving antimicrobial stewardship requires attention to knowledge, interactions, and relationships between clinical teams and infrastructure that supports stewardship and team interactions.