This mutually collaborative action research aimed to develop age-friendly primary health care (AFPHC) for older people at one primary care unit (PCU), under the supervision of the university hospital, Khon Kaen province, Thailand. The participants were older people and their family members, nurses, the staff of the primary care unit, community health volunteers (CHVs), and community leaders. Data were collected by participatory observations, natural interviews, in-depth interviews, focus group discussions, and document reviews. Qualitative data were analyzed by content analysis. Quantitative data were analyzed by using frequencies and percentages. The findings of six concepts of AFPHC: Respect, direct services, equity in elders group, good death, family care, and age-friendly environment, led to the process of developing age-friendly primary health care for the elders in four phases. These were: 1) awareness of AFPHC concepts, 2) changes in health service behaviors and age-friendly environments, 3) developing age-friendly services and activities, and 4) building age-friendly service networks. The results of this study suggested that age-friendly primary care comprised of three components: age-friendly behaviors, age-friendly services, and an age-friendly environment. Factors contributing to the development of AFPHC were: 1) positive thinking of participants, 2) management for mutual benefits, 3) social capital of local stakeholders, and 4) input from gerontological advanced practice nurses in the community. Barriers to the development of AFPHC were: 1) the changed policies of the PCU administration, 2) the personnel outcome evaluation system that focused more on individual than groups or project based.