Though it has passed over 30 years, Korea’s community-based health insurance (CBHI) expansion can provide useful policy implications to developing countries with similar conditions, that is, lack of fiscal resources, health infrastructure, and medical resources to expand coverage to the informal sector. We summarized three groups of success factors through in-depth interviews and narrative analysis: system design, system operation, and public perception of the system. Korean CBHI could expand to the informal sector with the same system design as the formal sector such as mandatory enrolment, compulsory designation of medical service providers along with the low-benefit, low-contribution, and a low-payment system. However, expansion to the informal sector was somewhat different, as the CBHI exercised and operated the scheme with flexibility, semi-autonomy and leadership to fit for local context in terms of operation. Moreover, cultural factors that encouraged public awareness and increased participation significantly contributed in appealing to the informal sector. Overall, the systemic, operational, and cultural factors interacted with each other and created a synergy effect that local members in the informal sector found attractive.