This paper presents the development of a comprehensive mixed-method instrument designed to measure Environmental, Social, and Governance (ESG) disclosure among Malaysian Public Listed Companies (PLCs). Recognising the increasing regulatory emphasis on sustainability reporting under frameworks such as the Malaysian Code of Corporate Governance (MCCG) and the National Sustainability Reporting Framework (NSRF), the study adopts both quantitative and qualitative approaches to ensure methodological robustness and contextual relevance. The quantitative component employs a binary-scoring ESG disclosure checklist (1 = disclosed; 0 = not disclosed) to objectively assess the extent of ESG reporting across corporate annual and sustainability reports, supported by experts validation. Complementing this, the qualitative component utilises semi-structured interviews with key informants—such as regulatory officers, governance experts and internal auditors—selected through purposive sampling until data saturation (12–13 participants) to gain deeper insights into disclosure motivations, regulatory influence, and implementation challenges. Integrating both approaches allows for triangulation of findings, linking quantitative disclosure outcomes with qualitative interpretations of organisational behaviour. The methodological framework proposed in this study provides a rigorous, adaptable tool for assessing ESG disclosure quality in emerging markets, offering valuable implications for researchers, policymakers, and practitioners seeking to enhance corporate transparency and governance accountability in Malaysia.