MacMullen was commissioned a second-lieutenant on the unattached list of the Indian Army on 4 August 1897, and served on the North West Frontier in 1897. Promoted to lieutenant on 4 November 1899, he was with the 15th Bengal Infantry in 1900, and then with the Tibet Expedition in 1903. He saw action in World War I as a General Staff Officer Grade 1 with the 2nd Mounted Division during the Gallipoli Campaign and then as Brigadier-General on the General Staff with XV Corps in France.