He was a member of the Indian Army attached to the 3rd Fusilliers, and passed into the Indian Army at outbreak of war from Sandhurst College. His brother, 2nd-Lieutenant J A Miller-Haller had obtained his commission in the 11th Battalion South Wales Borderers and was stationed at Colwyn Bay.
J L Miller-Hallett was twice wounded in action on the Western Front.
First in June 1915, when it was reported in the Bromley & District Times that he had been wounded, but not very seriously, and had continued for a couple of days before getting the wound attended to and that he was now doing well. He was the only soldier at that fighting who was not either killed or rendered out of action due to injury.
On the 7th January 1916 it another report stated that as he was going out to join his regiment in India, his boat, the Persia, was torpedoed. The P&O liner Persia was the first passenger ship to be torpedoed without warning during WW1. She sank in December 1915 – hundreds of passengers lost their lives.
He was among those rescued and taken to Alexandria.