He enlisted in the Royal Canadian Air Force in May 1940 and was posted to the No.2 Service Flying Training School near Ottawa, Ontario. He was accepted for air crew training in December 1941 and flew Fleet Finch, Harvard and Yale single engine aircraft until he received his wings in September 1942.

He spent a year as a Flying Instructor before being posted overseas in March 1944. It was there he began training on Hurricanes and Spitfires, which he flew until the end of the war.

Leslie enlisted with the Royal Canadian Air Force (RCAF) in 1940, and although he had all the requirements for air crew and had passed the medical, he failed the Colour Vision Test which classified him for ground crew. After attending the RCAF Technical School near St. Thomas, Ontario he was posted to No.2 Service Flying Training School (SFTS) at Uplands just outside of Ottawa, Ontario.

The RCAF later developed the "Colour Vision Aperture Test" which Leslie took at RCAF Headquarters in Ottawa. He passed this test which placed him in the category, "Colour Vision Redgreen Defective, safe for air crew". His application for remuster to air crew was approved December 7, 1941 (Pearl Harbour Day).

After receiving his initial training at Belleville, Ontario, Leslie was posted to No.22 Elementary Flight Training School (EFTS) in Ancienne Lorette, Quebec flying the Fleet Finch II. This was followed by a posting to No.1 SFTS at Camp Borden. Ontario. flying Harvards and the Yale trainer, where he received his wings in September 1942. Leslie was posted to No.3 Flying Instructors School (FIS) at Arnprior, Ontario.

After completing his training at FIS, Leslie began instruction at No.6 EFTS at Prince Albert, Saskatchewan on the Tiger Moth. Later he was posted to No.19 EFTS at Virden, Manitoba. Leslie had remarked that although "instructing was not always that exciting, teaching young fellows how to fly before they learned to drive a car, was a rewarding experience."

In March 1944, Leslie was posted overseas to the Advanced Flying Unit (APU) at Ternhill, Shropshire, where he began his training on Miles Master and Hawker Hurricane aircraft. He was next posted to the Operations Training Unit (OTU) at Kirton, Lincolnshire where he flew the Spitfire VB.

The following year, in March 1945, Leslie was assigned to Squadron 403. He immediately left for Eindhoven, Netherlands where he joined Squadron Wing 127 flying the Spitfire XVI.

The 127 Wing consisted of the four squadrons, 403, 416, 421 and 443 and were tasked with supporting the British Second Army. In April, 1945 the 127 Wing occupied the German air field at Diepholz, and then moved to another area just south of Hamburg at the end of April, where they were stationed when the war ended on May 8th, 1945. Leslie and the rest of the 403 Squadron returned to England in July, 1945.

By miraculous good fortune, the Spitfire KH-Z (TB752) which Leslie flew with 403 Squadron survived the war. It was completely restored and is now housed in the Memorial Building at Manston Airport near Ramsgate, Kent.

After Leslie returned to Canada, he attended the University of Alberta and graduated with a BSc in Chemical Engineering in 1950. He worked in oil and gas exploration and production, and spent the last 27 years with the Hudson's Bay Oil and Gas Company Ltd in Calgary, Alberta. He retired with the position of Manager, Technical Services, and commenced a farming operation near Red Deer, Alberta.

During the war, while Leslie was a patient at the Canadian General Hospital in England, he met Lieutenant Nursing Sister Pauline Boon. Six years later they met again and were married shortly after. They raised a family of four children; Lorna, Keith, Christine and Sheila.