Saint Cyr was commissioned as an officer in Svea Artillery Regiment (A 1) with the rank of Fänrik in 1924. He attended the Artillery and Engineering College from 1926 to 1928 when he was promoted to Lieutenant. Saint Cyr then attended the Army Riding School from 1928 to 1929, from 1931 to 1932 and from 1934 to 1936. He served as a teacher at the Army Riding School from 1936 to 1940 and he was promoted to Captain in 1938. Saint Cyr became Major in 1944 and attended the Cavalry School in France from 1948 to 1950. He was placed in the reserve in 1957.
