ESG is a sustainable development concept that integrates environmental, social, and corporate governance. Most studies on ESG have been conducted based on secondary data from listed companies and have not used questionnaires as a method for analysis. Given this research gap, this paper examines whether transformational leadership influences ESG performance in SMEs, whether organizational innovation mediates the relationship between transformational leadership and ESG performance, and the moderating effect of external social capital on transformational leadership and organizational innovation. Based on higher-order theory, resource-based theory, stakeholder theory, etc., we tested this hypothesis by conducting a regression analysis with a questionnaire collected from SMEs in China. After controlling for firm ownership, firm size, firm industry, and years in business, the results of the study indicate that transformational leadership has a positive effect on ESG performance and that organizational innovation partially mediates the relationship between transformational leadership and corporate ESG performance. Furthermore, external social capital moderates the direct relationship between transformational leadership and organizational innovation and moderates the role of organizational innovation as a mediator between transformational leadership and ESG performance. This study adds to our further understanding of the relationship between transformational leadership and ESG performance in SMEs, expanding the antecedent research on ESG performance and providing a basis for sustainable SME development.