In September 1941, with the Second World War underway, McDiarmid played Britain's eight-time Grand Slam tennis champion
Fred Perry at an exhibition match at the Rideau Lawn Tennis Club. Hundreds of fans attended the wartime Red Cross fundraiser where McDiarmid played two matches against the reigning World's Professional Champion. In the afternoon, McDiarmid took the first set 6–4, and Perry took the second 6–3. In the evening, Perry bested McDiarmid 6–3, 6–3. In 1938, the McDiarmid family established the popular annual McDiarmid brothers' doubles tournament that rotated through Ottawa-area clubs. Don, Gordon, Malcolm, and Clifford competed each year before family and friends. An article in the
Ottawa Citizen in November 1942 reported that the unique family tournament was cancelled for the duration of the war, noting that Corporal McDiarmid had deployed to England with the Royal Canadian Air Force. Having enlisted in the spring of 1942, McDiarmid deployed overseas in November as part of the RCAF's clerical establishment. He quickly became a sports ambassador for Canada and the
Royal Canadian Air Force. He was initially stationed in London, where he was welcomed as a guest player at the Queen's Club with full membership privileges. After London, he had short postings to Prestwick, Scotland, and the
Inner Hebrides on Scotland's west coast. In 1943, he was posted to Algiers in North Africa, and then to Cairo, where he spent the balance of the war. In Cairo, McDiarmid played tennis at the Wilcox Sports Club, winning several club titles. He also played a state tennis tournament, losing in the semi-finals to ace Egyptian player Andre Najar. During his leave period after the war in 1945, McDiarmid visited Athens, Naples, Glasgow, and spent a week in Edinburgh, playing tennis at clubs in each of those cities. In late September 1945, McDiarmid was invited to play at a Canadian military services tournament in Hilversum, a small city in the Netherlands 24 kilometres southwest of Amsterdam (Canadian forces played a major role in liberating the Netherlands from Nazi occupation). Sergeant McDiarmid won his match in three straight sets, winning the singles trophy and helping the RCAF win the overall team trophy, beating the two army teams. McDiarmid sailed back to Canada in October 1945 in the troopship Queen Elizabeth. Due to his confirmed sailing date, he had to turn down an invitation to play tennis at Wimbledon. (As Canadian open champion in 1940, he qualified to play at Wimbledon, which was suspended from 1940 to 1945 due to the war. He never played at Wimbledon). == Post-War tennis career ==