Thailand is a popular host nation for international migrant workers, particularly those from Cambodia, Lao PDR, and Myanmar. Thailand has introduced approaches to protect their rights for health and social welfare, using various mechanisms over many years. However, the implementation of these policies is dynamic and has been influenced by national security, economic necessity, and public health concerns. The aim of this study was to explore how Thailand designs and implements health and social welfare policies for migrants in Thailand, both before and during COVID-19. A qualitative analysis was used alongside interviews with 18 key informants in various sectors in this field. Thematic coding was applied. Results show that there were seven key themes emerging from the analysis, including: (i) sustainability of the HICS; (ii) people dropping out from the Social Security Scheme (SSS); (iii) quality of health screening in the Memorandum of Understanding (MOU) migrants; (iv) health screening problems and state quarantine management in response to COVID-19; (v) managing the migration quota and dependency on migrant workers; (vi) influx of migrants in the backdrop of COVID-19; and (vii) poor living conditions of migrants and the impact of COVID-19. The majority of interviewees agreed that undocumented migrants is a critical concern that impedes access to migrants’ health and social welfare. This situation was especially pronounced during the second wave of COVID-19 in Thailand, which took hold in migrant communities. In the short term, the poor living conditions of migrants urgently need to be addressed in order to contain and mitigate this crisis. In the long term, there needs to be an improved health system design that includes migrants, regardless of their immigration status. This requires intersectoral policy coherence, including the hastening of nationality verification to sustainably mitigate undocumented migrants.