This article explores the early life of Eldridge Cleaver and his rise to prominence within the Black Panther Party. It argues that historians have largely overlooked the importance of Cleaver’s radicalization while incarcerated inside California’s penitentiaries between 1954 and 1966; this experience deeply influenced his rise to prominence as a leading figure in the Black Panther Party following his parole. The article details his earliest contacts with politically active inmates during the mid-1950s, who introduced Cleaver to radical political literature. He subsequently joined the Nation of Islam temple within San Quentin penitentiary and rose through the ranks to become the temple’s minister. Through the use of Cleaver’s papers, held by the Bancroft Library at the University of California Berkeley, this article contributes to the existing historiography on Cleaver in three ways. Firstly, it connects Cleaver’s membership of the NOI with the growing politicization of African American inmates during the late 1950s and early 1960s. Secondly, it highlights the NOI’s success in recruiting prisoners and the brutal repression that was inflicted upon members of the Nation of Islam by prison staff. It argues that Cleaver’s experience of fighting prison racism was crucial in shaping his identity as a black nationalist, and continued to influence his position in the Black Panther Party after his release.