Frontier Internship in Mission (FIM) was an experimental mission project conceived of and run by Presbyterian Student World Relations director Margaret Flory between 1961 and 1974. It was broadly ecumenical in concept and execution and closely tied to the World Student Christian Federation community. Recent college or seminary graduates were assigned to live and work with local people who were connected in some way to the global ecumenical network and who had invited them. They worked on projects mutually agreed upon, usually for two years. One hundred fourteen of the 140 Americans who originally participated and eight of the original 20 international participants were interviewed for this study. Their narratives about their life histories and experience during and after these international partnerships offer an intimate look at one group of largely mainline Protestant Americans born in the 1930s and 1940s, and the social and religious institutions that were their avenues to engagement with the wider world at a time of cataclysmic change. Over the thirteen years of FIM program operation considered here, conditions in the forty-eight different countries where Frontier Interns (FIs) served were transformed by movements for independence and by escalating covert and overt American intrusions. The core of this dissertation presents regionally-organized internship case studies highlighting the impact of those encounters on the FI’s Christian and American identities . It also analyzes the rejection of their witness when they returned home. Moving forward with their lives, Frontier Interns reaffirmed their commitment to “right relations” of mutual respect across difference and most often gravitated to social roles as bridge-builders and interpreters, domestically and internationally. The strong internal opposition to global ecumenism that had developed in some mainline Protestant churches changed the relationship of many FIs to those churches. It is argued here that the Frontier Interns’ experience highlights a societal shift from a moral order based on covenant or social contract to one that privileged the unrestrained exercise of power and interests. A covenantal commitment to mutual global partnerships is central to who the FIs are, their internships, and what they did with their lives subsequently.