Abstract: In the social sciences, belongingness is among the individual core social needs. Institutionalization is a powerful mediator in the individual search for belongingness, as it establishes agency among group members. Using a conceptual framework that borrows from theories of institutionalization and group membership, we examine the lives of incarcerated military veterans and their choices in group affiliation during incarceration, especially in the context of American Legion posts behind bars. We use interview data from 43 incarcerated veterans to interpret patterns of belongingness, group trust, and group affiliation as developed within and across institutions. Five themes related to American Legion involvement emerged: (1) brotherhood and camaraderie; (2) serving others and the community; (3) creating a routine and improving quality of life; (4) creating community; and (5) leadership and conflict. Research on belongingness, group membership, and participation in peer-led prison programs is critical for improving knowledge on prison adaptation. It also provides a foundation for innovative approaches to post-release integration and desistance from crime among justice-involved veterans.