He served with the Royal Australian Air Force flying Meteors in a ground attack role during the Korean War.

He was Chief of the Air Staff during the early 1980s at the time of the emergency airlift of food and supplies to Ethiopia ("Operation Bushel").

[10] He was awarded the Air Force Cross in the 1968 Birthday Honours,[11] and promoted to group captain on 1 July 1968.

[13] Promoted to air vice marshal on 1 July 1975,[14] he was appointed Commandant of the RAF Staff College, Bracknell, on 16 August 1975.

[18] Williamson was made Commander-in-Chief Strike Command with the acting rank of air chief marshal on 15 September 1980.

[19] He was promoted to the substantive rank of air chief marshal on 1 March 1981[20] and advanced to Knight Grand Cross of the Order of the Bath in the 1982 Birthday Honours.

[24] He also implemented the emergency airlift of food and supplies to Ethiopia in the wake of severe drought, famine and civil war there ("Operation Bushel") and advised on the international agreement to proceed with the European Fighter programme.