Background: An on-site pharmacist (OSP) intervention was implemented which sought to improve medication management within residential aged care facilities (RACFs) in the Australian Capital Territory, Australia. The objectives of this mixed methods study were to evaluate the implementation fidelity of the OSP intervention and to determine the moderating factors which influenced delivery of this intervention. Methods: This convergent parallel mixed methods study was underpinned by Hasson’s conceptual framework for implementation fidelity. Implementation fidelity for seven intervention RACFs was quantitatively assessed using three quantitative data sets: (1) range of OSP intervention activities delivered; (2) random sample of 10% of medication reviews assessed for quality; (3) proportion of residents who received at least one medication review. Semi-structured interviews (n = 14) with managers and OSPs across the intervention RACFs were conducted to identify moderating factors which may have influenced OSP intervention delivery. Results: The OSP intervention was generally delivered as intended with overall medium levels of implementation fidelity. This delivery was supported by a range of facilitation strategies with most participants perceiving that the intervention was delivered to a high standard. RACF managers and OSPs were mostly well engaged and responsive. A number of potential barriers (including the part-time OSP role, COVID-19 pandemic, RACFs spread out over a large area with significant distance between resident dwellings) and facilitators (including the pharmacist support meetings, OSPs who took time to establish relationships, RACF managers who actively supported OSPs and worked with them) for OSP intervention delivery were identified which have potential implications for the roll out of OSPs within Australian RACFs. Conclusion: In this study, the implementation fidelity of OSP intervention delivery was assessed with overall medium levels of fidelity found across the intervention RACFs. This suggested that the OSP intervention can generally be delivered as intended in real world RACFs. OSP intervention delivery was influenced by a range of moderating factors, some of which posed barriers and others which facilitated the OSP intervention being delivered as intended.