Introduction: HL7 Fast Healthcare Interoperability Resources (FHIR) has become a leading global standard for health information exchange and is increasingly central to national digital health architecture. Sri Lanka has formally adopted HL7 FHIR Release 4 as its preferred national interoperability standard and has developed national implementation guides, governance mechanisms, and connectathon-based testing activities. However, the operational maturity of FHIR adoption across Sri Lanka’s health system has not previously been systematically assessed.Methods: A mixed-methods baseline assessment was conducted between September and November 2025, using a desk review, rapid literature review, technical review of national FHIR Implementation Guides, semi-structured key informant interviews (n=18), stakeholder surveys (n=16; response rate 88.9%), and interoperability maturity assessments. Interoperability maturity was assessed using the HL7 FHIR Maturity Model (FMM) and the MEASURE Health Information Systems Interoperability Maturity Toolkit. Qualitative data were analysed thematically, and candidate barriers and recommendations were validated with stakeholders using the Nominal Group Technique with Likert-scale scoring. Findings were triangulated across data sources.Results: Sri Lanka has established key foundations for standards-based interoperability, including national FHIR implementation guides, dedicated governance structures, national FHIR Connectathons, and pilot implementations that demonstrate FHIR-enabled data exchange. However, national FHIR artefacts remain at FMM Level 1, with profiles tested against only 37.5% of core data elements, which is below the 80% threshold required for FMM Level 2, while overall interoperability maturity was assessed as nascent (Level 1) across leadership and governance, human resources, and technology domains. Twenty-one barriers to adoption were identified, including limited workforce capacity, lack of national testing infrastructure, insufficient vendor incentives, fragmented governance, donor-dependent financing, and absence of procurement mandates for FHIR compliance. A phased roadmap and a multi-tier national FHIR governance model were derived to guide scaling.Conclusion: Sri Lanka has moved beyond policy endorsement of FHIR and has demonstrated early technical feasibility, but ecosystem-wide implementation remains nascent. The main challenge is no longer the selection of standards alone, but their institutionalization through sustained governance, financing, workforce development, conformance testing, procurement alignment, and regulatory mechanisms. The findings provide a baseline for Sri Lanka and offer transferable lessons for other low- and middle-income countries seeking to scale standards-based interoperability.