Background Systematic reviews have found limited evidence of effectiveness and impact of clinical librarians (CLs) due to the poor quality of reporting, scale and design of previous studies. Objectives To measure specific CL impact on organisational and patient outcomes using a robust approach that helps CLs develop research skills. Methods Questionnaire and interviews. Results Clinical librarians contribute to a wide range of outcomes in the short and longer term reflecting organisational priorities and objectives. These include direct contributions to choice of intervention (36%) diagnosis (26%) quality of life (25%), increased patient involvement in decision making (26%) and cost savings and risk management including avoiding tests, referrals, readmissions and reducing length of stay (28%). Discussion Interventions provided by CL's are complex and each contributes to multiple outcomes of importance to health care organisations. Conclusion This study is unique in taking a wide view of potential and specific impacts to which CLs contribute across health care organisations. It is the largest UK evaluation of CL services to date and demonstrates CLs affect direct patient care, improve quality and save money. Future researchers are urged to use the tools presented to collect data on the same outcomes to build a significant and comprehensive international evidence base about the effectiveness and impact of clinical librarian services.