At the time of Canada's entrance into World War II, Nicholson was serving as the principal at Battleford Collegiate. In 1940, at the age of 38, he joined the Canadian Army and was commissioned into the
Prince Albert and Battleford Volunteers, a
Non-Permanent Active Militia regiment. In April 1941 the Battleford component of the regiment was separated and converted to infantry, and the name of the remaining component reverted to the Prince Albert Volunteers. A year later, on 5 March 1942, the 1st Battalion, Prince Albert Volunteers was activated and transferred to Vernon Military Camp as part of the 19th Infantry Brigade. Nicholson, who by this time had moved to
Biggar, Saskatchewan, joined the battalion in mid-April. Upon his departure, a reporter for the
Saskatoon Star-Phoenix noted, "Mr. Nicholson will be missed in Biggar, as in a short time here he has taken part in public activities, serving as president of the Canadian Club, leader of St. Paul's Church choir, and taken an interest in the dramatic and musical life of the town. He had also been connected with military organization in Bigger and was in charge of the local unit of the Reserve Army. He was also instrumental in forming a cadet corp in Biggar." In 1943, Colonel
Archer Fortescue Duguid (1887–1976) of the Canadian Army Historical Section requested Nicholson's transfer to Ottawa to join the Section. This request was based in part on a confidential report on Nicholson's character that Duguid had read. The report described Nicholson as :a confident, inspiring leader, poised, reasonably aggressive and self-assured [...] He is an excellent lecturer, which is backed up by a fine educational background and educational training as a High School Principal. He is a hard and conscientious worker and a good learner. Quite original, resourceful, and has an excellent ability. He has a charming, agreeable personality, optimistic nature, cheerful outlook [...] An artistic nature, rather than practical; is sympathetic, tactful, diplomatic, loyal, co-operative, high spirituality and sense of duty. Shortly after his appointment to the Historical Section, Nicholson was appointed as the narrator for Pacific Command. In the wake of
Operation Cottage in August 1943, he traveled in U. S. Army aircraft through the
Aleutian Islands. While covering the Pacific, Nicholson was a passenger in the first American plane to land on
Kiska after its capture from the Japanese. Following the cessation of hostilities in September 1945, in early 1946, new Director of the Historical Section Colonel C. P. Stacey (1906–1989) appointed Nicholson the Officer-in-Charge of the Historical Section at Canadian Military Headquarters in London. During his time in London, Nicholson gathered documents pertaining to Canada's role in the
Battle of Hong Kong of December 1941. This work resulted in the 1946 pamphlet entitled
Canadian Participation in the Defence of Hong Kong, December 1941. In 1947, Nicholson returned to Canada where he was appointed Deputy Director of the Historical Section, Army Headquarters, Ottawa. During his time with the section, Nicholson authored two full-length books. The first was
Marlborough and the War of the Spanish Succession, published in 1955. His next work, published a year later, was the second in the three-volume
Official History of the Canadian Army in the Second World War. Nicholson's volume,
The Canadians in Italy, 1943–1945, dealt exclusively with the
Italian Campaign. On 1 August 1959, Nicholson took over from Col. Stacey as the director of the Historical Section. At this time he was promoted from Lieutenant-Colonel to Colonel. == Retirement and later years ==