William was commissioned as a 2nd Lieutenant to the 8th Battalion Worcestershire Regiment on the 1st January 1909.
On the 22nd June 1910 he gained the rank of Lieutenant and on the 26th September 1914 promoted to rank of Captain, still with the 8th Battalion Worcestershire Regiment.
On the 31st March 1915 he embarked at Folkestone for France with the rest of the 8th Battalion reaching Boulogne later that same night. The following year (21/04/1916) he was seconded for staff duties with the British Expeditionary Force and on the 16th July 1917 appointed as D.A.Q.M.G (Deputy Assistant Quartermaster General). On the 13th February 1918 he attained the rank of Major, still seconded on staff duties with the Headquarters of the 4th Army.
For his actions in France in 1917 he was awarded the Military Cross (London Gazette date 04/06/1917). In the 1918 Kings New Years honours he received the O.B.E (London gazette date 01/01/1918).
He was mention in despatches no less than 3 times and on the 17th March 1920 was given the French Croix de Guerre.
In November 1918 he rejoined the Worcestershire Regiment.
On the 26th January 1923 he was made Lieut.-Colonel and took command of the 8th Battalion (Defence Force) Worcestershire Regiment.
Lieut.-Colonel W. E. L. Cotton
The 8th Battalion at Swanage (Annual Training) August 1924
Left to Right: Major J. M. Reddie, C.B., D.L., Colonel R. C. MacCalmont,
D.S.O. (comdg. 144th Inf. Bde.), Major Lord Cobham (Lord Lieut. of
Coventry), Lieut.-Col. W. E. L. Cotton, O.B.E, M.C., T.D. (comdg. 8th Bn.),
Captain S. A. Gabb, O.B.E., M.C. (Adjutant 8th Bn.) and Lieut. H. Heath, M.C.
(Quartermaster 8th Bn.)