In 1904 Carl Gustaf Lewenhaupt became a lieutenant with the dragoons of the King’s own regiment. In 1909-11, he studied at the National Defence College, and was a candidate for the General Staff from 1912-14. He also studied at the French Riding School in Saumur and the French General Staff, and served in the Belgian Army in 1913, and was later promoted to major. After that, he was head of the railway police for several years and, concurrently, secretary of the Jockey Club.
In 1912, 1920 and 1924, Lewenhaupt was a member of the Swedish Olympic Committee for Equestrian Sports and represented Sweden at the Olympic Congresses in Lausanne and Paris. In 1920 he won a bronze medal in individual jumping on his horse Mon Coeur. At Paris in 1924 he was not able to finish the competition but did win a silver medal in team eventing. Lewenhaupt was also a founding member of the Brunkeberg Club, a Swedish nationalist political association.