Background: Primary care (PC) reform is imperative to meet the demands of a rising number of chronically ill patients with complex needs. Patient-Centered Medical Home (PCMH) is a new care model that was found to improve care for complex needs patients in some countries but has not yet been widely adopted in Singapore. Guided by the PCMH principles, “ComSA-PCMH” was developed to proactively deliver integrated PC to a specific population with complex needs. This study explored the change strategies, initial experience and perception, and lessons learnt during its early implementation.Methods: A grounded theory approach was employed. In-depth interviews were conducted for twenty-two key informants from three groups: the implementers, their implementation partners and other PC providers. The theoretical framework on diffusion of innovations by Greenhalgh and colleagues (2015) was used to determine theoretical saturation, reorganize data, and provide insights to the emerging themes.Results: “Diffusion of innovation” emerged as an overarching theory to contextualize ComSA-PCMH and its early implementation. ComSA-PCMH was differentiated from usual PC through three innovations: i) team-based and integrated care; ii) empanelment; and iii) shared care with private general practitioners. Their corresponding change strategies were: i) repurposing pre-existing services and infrastructure; ii) partnership to create supporting infrastructure and pathways in the delivery system; and iii) targeted outreach. Initial experience of the implementers was characterized by “assimilation”, which was a cyclical process of experimentation, negotiations, and adaptations. Initial perception of the implementation partners was characterized by “adoption”, which was affected by perceived value of ComSA-PCMH and perceived burden in implementation. Initial perception of the private general practitioners was also characterized by adoption, which was affected by a lack of awareness and understanding of ComSA-PCMH. Lessons were learnt about ways to work with the complexity and novelty of the innovations.Conclusions: ComSA-PCMH employed creative and pragmatic strategies to overcome country-specific contextual challenges and the inherent complexities of the care model. Assimilation and adoption were identified as the challenging steps, as they involve complex processes participated by multiple players who might exhibit less predictable, self-organizing behaviors.Trial Registration: This study was retrospectively registered with ClinicalTrials.gov (Protocol ID: NCT04594967).