Born in County Wicklow, Ireland in 1864, Wanless O'Gowan joined the British Army as a lieutenant in the 8th (Militia) Battalion of the King's Royal Rifle Corps (KRRC) on 13 October 1883.

[5] He was sent to South Africa in 1899, the year the war started, and was severely wounded in action at the Battle of Spion Kop; he later served as a railway staff officer in 1900 and 1901 before returning home with the brevet rank of major, to which he had been promoted in November 1900.

[14] O'Gowan was then promoted to temporary brigadier general and transferred to a field command, taking over the 13th Infantry Brigade in the 5th Division from Edward Cooper.

At the Somme, the division was assigned to capture Serre on the flank of the main assault and guard against counter-attacks; however, the attacking battalions were decimated by German machine-guns before crossing no-man's land, with only small groups surviving to reach the far trenches.

[20] In April 1918 he returned to England to assume command of the Cannock Chase Reserve Centre, taking over from Major General Richard Hutton Davies, who had been relieved due to severe physical and mental illness, and was allowed to retain his temporary rank.