US Navy hospital ships are used as a foreign policy instrument to achieve various objectives that include building partnerships. Despite substantial resource investment by the Department of Defense (DoD) in these missions, their impact is unclear. The purpose of this study was to understand how and why hospital ship missions influence partnerships among the different participants. An embedded case study was used and included the hospital ship Mercy's mission to Timor-Leste in 2008 and 2010 with four units of analysis: the US government, partner nation, host nation, and nongovernmental organizations. Key stakeholders representing each unit were interviewed using open-ended questions that explored the experiences of each participant and their organization. Findings were analyzed using a priori domains from a proposed partnership theoretical framework. A documentary review of key policy, guidance, and planning documents was also conducted. Fifteen themes related to how and why hospital ship missions influence partnerships emerged from the 37 interviews and documentary review. The five most prominent included: developing relationships, developing new perspectives, sharing resources, understanding partner constraints, and developing credibility. Facilitators to joining the mission included partner nations seeking a regional presence and senior executive relationships. Enablers included historical relationships and host nation receptivity. The primary barrier to joining was the military leading the mission. Internal constraints included the short mission duration, participant resentment, and lack of personnel continuity. External constraints included low host nation and United States Agency for International Development (USAID) capacity. The research finds the idea of building partnerships exists among most units of analysis. However, the results show a delay in downstream effects of generating action and impact among the participants. Without a common partnership definition and policy, guidance, and planning documents reinforcing these constructs, achieving the partnership goal will remain challenging. Efforts should be made to magnify the facilitators and enablers while developing mitigation strategies for the barriers and constraints. This is the first study to scientifically assess the partnership impact of hospital ship missions and could support the DoD's effort to establish, enable, and sustain meaningful partnerships. Application of the findings to improve partnerships in contexts beyond hospital ship missions may be warranted and require further analysis. This unique opportunity could bridge the rift with humanitarian actors and establish, enable, and sustain meaningful partnerships with the DoD.