Diamond began doing sit-ins at the age of 15, in Petersburg, Virginia with groups and by himself. He would reportedly take a stool at segregated counters and wait to be served until the managers called the Petersburg Police Department, at which point he would run for the door.Notably, he was also a part of the sit-in movement in Washington, DC in the 1960s. At the age of 19, he engaged in protests at People's Drug Store at Lee Highway and Old Dominion Drive in Arlington Virginia. Following this particular sit-in, Diamond and others moved the sit-in to the Drug Fair at 5401 Lee Highway, where they were also refused service.One of the more noted sit-in's Dion was part of occurred in Arlington, Virginia at a drugstore on June 9, 1960. Diamond and his classmates at Howard University formed a group called the Nonviolent Action Group from which he participated in protests around Arlington.He also was a part of the Freedom Riders and was a field secretary for the Student Non-Violent Coordinating Committee (NAG) in Mississippi and Louisiana from 1961-63.