He led the 173rd Airborne Brigade, the first US Army unit to deploy to South Vietnam and later commanded the 25th Infantry Division there.

[1] During high school he joined the 120th Infantry Regiment, part of the North Carolina National Guard.

[2] Williamson was commissioned as a Second Lieutenant in March 1941 and with the US entry into World War II, the 120th Infantry was federalised and sent for overseas service.

[5]: 26 [6] From 28 June to 2 August the 173rd conducted a sweep operation near the Rung Sat Special Zone which failed to locate any Viet Cong.

[5]: 32–3 In November 1965, the 173rd launched Operation Hump at the conclusion of which the unit claimed that they had killed over 400 Vietcong, a figure which was acknowledged to be a guess by Williamson given under pressure from General Westmoreland at Military Assistance Command, Vietnam.

"[6]: 104 In February 1966, Williamson was appointed assistant commander of United States Army Infantry School at Fort Benning, Georgia.

He was later promoted to major general and on 1 November 1966 was given command of the Advanced Infantry Training Center ("Tigerland") at Fort Polk, Louisiana.