BACKGROUND: A new on-site pharmacist (OSP) intervention within residential aged care facilities (RACFs) is being investigated to help improve medication management. Interprofessional collaboration amongst prescribers, nursing staff and pharmacists is critical to improving RACF medication management. OBJECTIVE: To explore the extent of interprofessional collaboration and the nature of the working relationships between OSPs and prescribers, managers and nursing staff. METHODS: A mixed-methods study was undertaken within the context of a 12-month cluster randomised controlled trial. Semi-structured interviews were conducted with data analysed using framework analysis, and a survey based upon the Physician-Pharmacist Collaboration Index (PPCI) was distributed at two time points (T1 at 3 months and T2 at 9 months after OSP commencement) across seven intervention RACFs. RESULTS: The qualitative data (n = 33 interviews) findings related to the processes supportive of these relationships e.g. on-site proximity, OSP personality and perceived (or beneficial) benefits of OSPs working with health care team members (such as OSPs being trusted and providing reassurance to RACF health care team members). The PPCI survey mean scores at T1 (n = 33) and T2 (n = 19) suggested that OSPs were able to establish positive working relationships at 3 months and that positive relationships also existed at 9 months. The integrated findings suggested that the working relationships between OSPs and health care team members were generally positive. CONCLUSIONS: This study is the first to explore interprofessional collaboration between OSPs and health-care team members in RACFs. The findings suggest that OSPs can positively contribute to interprofessional collaborative care within RACFs.