Purpose – The study investigates how CEO narcissism influences a firm’s Environmental, Social, and Governance (ESG) performance and its overall market value. Furthermore, it explores whether ESG performance acts as a mediating variable between narcissistic leadership and firm value within the specific context of Indonesia, Malaysia, and Thailand (IMT countries). Design/methodology/approach – The research employs a quantitative approach using secondary data from publicly listed companies in Indonesia, Malaysia, and Thailand. CEO narcissism is measured through objective proxies (such as signature size or photo prominence in annual reports), while ESG scores and Tobin’s Q (for firm value) are retrieved from financial databases. The study utilizes panel data regression and mediation analysis to test the hypotheses. Findings – The results indicate that CEO narcissism has a significant impact on ESG disclosure and firm value. Specifically, narcissistic CEOs may drive higher ESG scores to seek external validation and "celebrity" status, though the long-term impact on firm value varies across the IMT region. The mediation analysis reveals the extent to which ESG initiatives serve as a strategic tool for narcissistic leaders to enhance corporate reputation. Research limitations/implications - The study is limited to the IMT region, which may possess unique cultural and regulatory characteristics (such as family-owned business dominance) that affect the generalizability of the findings to Western markets. Additionally, the use of archival proxies for narcissism, while objective, may not capture the full psychological depth of a leader compared to primary psychometric assessments. Practical implications – For boards of directors and investors, this research highlights the importance of "narcissism-proofing" corporate governance. It suggests that while narcissistic CEOs can be catalysts for aggressive ESG adoption, their motivations must be aligned with long-term value creation rather than short-term personal branding. It provides a framework for better executive selection and monitoring in the Southeast Asian context. Originality/value – This research contributes to the Upper Echelons Theory by examining personality traits in the emerging market context of IMT countries. It is one of the few studies to bridge the gap between behavioral psychology (narcissism) and sustainable finance (ESG) in Southeast Asia, offering fresh insights into how leader personality shapes corporate sustainability agendas.