Born in Australia, but of British and Hungarian descent, Leslie served during the Boer war, where he was awarded the Victoria Cross. Reporting the actions which led to the award, the London Gazette said:

“On 23 November 1901 at Geelhoutboom, Natal, Maygar galloped out and ordered men of a detached post, which was being outflanked, to retire. The horse of one of the men was shot under him when the enemy were within 200 yards and he dismounted and lifted the man on to his own horse which bolted into boggy ground, making them both dismount. As the horse could not carry two, Maygar again put the man on its back and told him to gallop for cover at once, while he himself went on foot. All this took place under very heavy fire.”

Following the outbreak of the First World War, being overage, Leslie lied about his date of birth to enlist. He served at Gallipoli in 1915.

He became commander of the 8th Australian Light Horse and served in the Sinai and Palestine Campaign, temporarily commanding the 3rd Light Horse Brigade on three occasions. He was awarded the Distinguished Service Order (DSO) in June 1917, and qualified for the Volunteer Officers' Decoration (VD) in July. Leslie was wounded during the Battle of Beersheba by a German aircraft on 31 October 1917 and died in hospital in Karm, Palestine, on 1 November.

He is buried in CWGC Beersheba War Cemetery, Q. 82.