McMahon was appointed chief instructor of the army's School of Musketry in 1905 and sought to improve the infantry's rate of fire.

His request for additional machine guns to be allocated to each battalion was rebuffed and he instead worked on increasing the rate of fire from the riflemen.

McMahon was promoted to brigadier-general in the first months of the war and was due to take over command of the 10th Infantry Brigade shortly before he was killed by shell fire.

[12][13] McMahon returned to service later in the war and held the appointment of deputy assistant adjutant general (DAAG) from February to June 1902.

[3][14][15] As chief instructor in 1907 he foresaw the future of warfare and recommended that each infantry battalion be allocated 6 machine guns.

Under McMahon all infantrymen were required to be able to fire 15 aimed shots in a minute, a practice that was enshrined in the Musketry Regulations of 1909.

[17] McMahon relinquished his appointment as chief instructor on 9 June 1909 and thereafter served at army headquarters until January 1910.

[3][18] After the outbreak of the First World War in August 1914 McMahon served with the Royal Fusiliers on the Western Front and was mentioned in dispatches on 8 October 1914 by Field Marshal Sir John French, commander-in-chief (C-in-C) of the British Expeditionary Force (BEF), for his actions during the First Battle of the Aisne.