William Marvin Watson was born on June 6, 1924, in Oakhurst, Texas, and attended Baylor University. When the United States entered World War II, Watson left school to serve in the Pacific with the Marines. Following the conclusion of the war, with the help of the G.I. Bill, Watson returned to Baylor, where he received his master's in business administration.

After graduation, Watson became the executive assistant to E. B. Germany, the president of Lone Star Steel Company. He also was chosen, in 1958, as a member of the Texas State Democratic Committee. While serving in that capacity, Watson created one of the first Johnson-for-President clubs in Texas; as a result of his early work on Johnson's behalf, Watson was asked to serve as Johnson's convention coordinator. After Johnson was elected President in 1965, Watson was appointed White House special assistant.

Watson was Johnson's appointments secretary and served as liaison with Democratic governors, big-city mayors, the Federal Bureau of Investigation, and the Democratic National Committee. His powers grew to include many of those assumed by the White House chief of staff after Bill D. Moyers resigned that position on December 14, 1966. Watson was also involved in helping President Johnson decide not to seek reelection.

In April 1968, acting on his belief that the postal system needed reform, Johnson appointed Watson as postmaster general. Watson created new regulations governing the mailing of firearms, which included identifying the contents and notification of such parcels before delivery. Watson retired from that post in January 1969.