There is little methodological guidance regarding how to best integrate qualitative observational case study data and quantitative large-N observational data in the study of public agencies in a mutually supportive way. There are a broad range of potential applications of mutually supportive mixed methods, which can be of help whenever one tool of inquiry (e.g., econometric analysis) suffers from weaknesses (e.g., omitted variables, measurement techniques which may not be unbiased, the inability to estimate important quantities of interest) to which another tool of inquiry (e.g., process tracing of case studies) does not. To demonstrate the broad relevance of mutually supportive mixed methods in public management scholarship, this article focuses on qualitative case studies as a way of addressing an econometric challenge of particular relevance to the field: accounting for the fixed features of units (e.g., agencies, or departments) in multiunit studies. The article’s central points are illustrated using mixed method data on foreign aid agency management practice and agency performance outcomes.