In 1957, while working for the National Council of Churches, Campbell participated in two notable events of the civil rights movement: he was one of four people who escorted the black students who integrated the Little Rock, Arkansas, public schools; and he was the only white person present at the founding of the Southern Christian Leadership Conference (SCLC) by Martin Luther King Jr.. Some black delegates opposed including him, but Bayard Rustin sponsored him. In 1961, he helped "Freedom Riders" of the Congress of Racial Equality (CORE) and the Student Nonviolent Coordinating Committee (SNCC) to integrate interstate bus travel, despite white mob violence, in Alabama. In a 1964 interview with Robert Penn Warren for the book "Who Speaks for the Negro?", Campbell discussed many of the issues of the civil rights movement, including the assassination of Medgar Evers by Byron De La Beckwith, desegregation busing, and the relationship between theology and social activism.