Leslie was a son of Ernest and Elizabeth Campbell of Inverell. He attended Ross Hill Primary School at Inverell. At the age of nineteen he enlisted at Holsworthy NSW in August 1915 and joined the 13th Battalion, 13th Reinforcement. He left Australia for Egypt on HMAT A60 Aeneas in December 1915.

After several months he was sent to France in June 1916. Early the following year he was appointed Lance Corporal. During August 1917 he was wounded in action and transferred to England for treatment at 5th Southern General Hospital, Portsmouth. He had a shell splinter in his eye and was eventually transferred to the Australian Army Medical Corps. He did not return to France. Leslie came back to Australia in 1918 and was medically discharged.

Leslie’s name is recorded on the Inverell Public School Honor Roll and the Inverell Honor Roll.