Henderson received his undergraduate and law degrees from University of California, Berkeley. In 1962, he became the Justice Department's first African-American lawyer in the Civil Rights Division. He was sent to the South to monitor local law enforcement for any civil rights abuses, a role that included investigating the 1963 16th Street Baptist Church bombing which killed four girls. In this capacity, he met Martin Luther King and other leaders of the Civil Rights Movement, after winning over their initial skepticism of being a government attorney.