He later joined 144th Brigade as Brigade Major and served with 144th Infantry Brigade from October 1915 to March 1916. He then served in the same capacity with 143rd Infantry Brigade from March to September 1916, before being appointed to the command of 3rd Battalion Worcestershire Regiment, with the rank of Acting Lieutenant-Colonel on the 5 September 1916.
He commanded the 3rd Battalion from September 1916 to April 1919, apart from 16th to 22nd September 1918 when he was required to temporarily assume command of the 57th Infantry Brigade. He joined his battalion again when they were in the line at Ovillers, Leipzig Salient and the Thiepval area. In October the battalion moved north, and wintered at Armentieres. He was to distinguish himself during the Capture of Westhoek, Ypres, during 10th to 12th August 1917.
In August 1917 he commanded both the 3rd Battalion Worcesters and the 13th Cheshire Regiment after all their senior commanding officers were wounded, he was awarded the D.S.O. for this action at Westhoek,Ypres in August 1917.
D.S.O. London Gazette 26 September 1917 (details published in London Gazette 9 January 1918):
"‘For conspicuous gallantry and devotion to duty. He took charge of a difficult and dangerous situation which had arisen owing to the right flanks of his division being exposed. Quickly grasping the state of affairs, he made excellent dispositions, and refused re-inforcements, although they were offered to him, acting with such determination and good judgement that from the moment he took over this sector there was never any doubt that the position would be maintained."
He was promoted Brevet-Lieut.-Colonel in 1919 for distinguished service in the field. He was twice mentioned in despatches.
After the war Whalley served as Brigade Major, Gloucestershire and Worcestershire Infantry Brigade, Southern Command from April 1920 to March 1922. He advanced to Brevet Lieutenant-Colonel and in November 1923 was posted for service with the 1st Battalion Worcestershire Regiment, at Meerut, India.