This article develops a framework and set of theories for understanding the formation, maintenance, and emergence of underground social movements—namely, those in which participants must remain hidden during a part of movement activity due to stigma, sanction, or laws, but depending on circumstances may exist partially hidden or fully “above ground” at various stages. It extends previous research on hidden and clandestine movements beyond a focus on those defined by extremist political ideologies, violence, revolutions, and wartime resistance, to include others based on stigmatized social identities and beliefs. Specifically, I draw on a ten-year qualitative case study from 2009 to 2019, consisting of sixty in-depth interviews and ethnographic observations of over 150 LGBT U.S. military service members who were secretly involved in some form of hidden mobilization in the repeal of various policies meant to exclude LGBT persons from openly serving in the military. I identify four different phases of movement formation—individual grievances, dyad creation, hidden networks, and movement emergence—and introduce the concept of “hidden distinctiveness” to capture how secrecy can become a strength for such movements.