As the world strives to achieve universal health coverage by 2030, countries must build robust healthcare systems founded on strong primary healthcare (PHC). In order to strengthen PHC, country governments need actionable guidance about how to implement health reform. Costa Rica is an example of a country that has taken concrete steps towards successfully improving PHC over the last two decades. In the 1990s, Costa Rica implemented three key reforms: governance restructuring, geographic empanelment, and multidisciplinary teams. To understand how Costa Rica implemented these reforms, we conducted a process evaluation based on a validated implementation science framework. We interviewed 39 key informants from across Costa Rica’s healthcare system in order to understand how these reforms were implemented. Using the Exploration Preparation Implementation Sustainment (EPIS) framework, we coded the results to identify Costa Rica’s key implementation strategies and explore underlying reasons for Costa Rica’s success as well as ongoing challenges. We found that Costa Rica implemented PHC reforms through strong leadership, a compelling vision and deliberate implementation strategies such as building on existing knowledge, resources and infrastructure; bringing together key stakeholders and engaging deeply with communities. These reforms have led to dramatic improvements in health outcomes in the past 25 years. Our in-depth analysis of Costa Rica’s specific implementation strategies offers tangible lessons and examples for other countries as they navigate the important but difficult work of strengthening PHC.