“Health in All Policies” (HiAP) had been acknowledged as a concept and principle in the National Health Act B.E. 2550 and implemented accordingly. There was still a gap of knowledge regarding the assessment of the implementation of HiAP through the National Health Act. This article aimed 1) to explore the institutionalization and implementation of HiAP concept in Thailand, and 2) to understand the foundations and development of the concept of Health in All Policy (HiAP) in the stream of health system development in Thailand. It aims for comprehensive understanding and for more effective interventions to achieve the goals guided by this concept. This study employed a qualitative research methodology. Data collection was collected from document review and in-depth interviews with 50 key informants, between April and July 2022. Thematic analysis was used to guide data analysis. HiAP emerged concretely through the promulgation of the National Health Act, B.E. 2550 through three key mechanisms, with the following findings: 1) Redefining health was clearly expressed in Chapter I Rights and Duties in Respect of Health of the Act and in the 2nd Charter on the National Health System. Intellectual health was an extraordinary dimension of ‘health’ that was defined by the World Health Organization. There were challenges concerning the promoting of the new health definition more acceptable and beneficial to the implementation of the Act, specifically the development of processes and formats for communicating new health definitions. 2) There were prominent mechanisms for enabling public engagement identified in the Act, namely the National Health Assembly process (Section 40, 44) and health impact assessment, HIA (Section 11). The National Health Assembly and HIA mechanism created an enhanced level of participation, networking, and learning. The challenge to make participation more meaningful was the authentic representation of the participants in the process and using a variety of methods to develop better policy issues. 3) Cross-sectoral collaboration appeared prominently in Section 2 of the National Health Board that identifies the state, national, local, social, academic components within and outside the health sector. In addition, the cross-sectoral collaboration designed in the Act aimed to achieve policy coordination through representatives from various sectors, making policy decisions legitimacy and in line with the direction of society. The challenge was the synergies between policy and impact. The key findings are that the foundations and developments of the HiAP concept are the results of the interrelated dynamic change in the global and national context of politics and health development. Furthermore, they relate to the emergence of organizations, mechanisms, and tools in the development of health systems from the Thai Ministry of Public Health and World Health Organization to the Second National Health Statute. Therefore, the effort to develop a health system by the HiAP approach needs to go beyond the scope of the National Health Act B.E. 2550 and its mechanism. Also, building connections and collaboration across national and international sectors is essential, as well as emphasizing the collective awareness that health is beyond the roles of public health organizations and the interventions focus mainly on the implicit concept of sanitary.