Jardine joined the British Army in August 1939. Once World War II began, he was commissioned into the Royal Berkshire Regiment and went with the British Expeditionary Force to France. He served at Dunkirk, where he was fortunate to escape but suffered some injuries. After serving as staff captain at St. Albans, he was posted to India for the remainder of the war. He served in Quetta, and then Simla, as a major. He became fluent in the Hindustani language. He gave lectures and played some cricket while in India. He left the army in 1945.