The United Nations introduced “Transforming our World: the 2030 Agenda for Sustainable Development” as a global action plan with 17 Sustainable Development Goals (SDGs) and 169 targets. In alignment with this agenda, the Stock Exchange of Thailand (SET) launched the Thailand Sustainability Investment Index (THSI) in 2015 to promote Environmental, Social, and Governance (ESG) factors. However, the number of sustainable companies has decreased, and Thailand’s economic competitiveness and foreign direct investment (FDI) have declined. This research thus aimed to develop guidelines for becoming a sustainable company using a mixed-methodology approach, including qualitative interviews, survey study, and focus group discussions. Quantitative data came from a survey of 500 sustainable companies listed on SET. The findings indicated that guidelines for SET-listed companies to achieve sustainability should consist of four latent variables, arranged in order of importance: organizational support, environmental protection, social responsibility, and corporate good governance. The most critical factor identified is the accurate and transparent reporting of financial performance. Additionally, the tested hypothesis indicated differences in market capitalization with overall aspects significantly different at the 0.05 level. These insights provide valuable strategies for businesses, policymakers, and educational institutions dedicated to advancing sustainable development and improving economic competitiveness in Thailand.