A native of Niigata Prefecture, Koyanagi was a graduate of the 42nd class of the Imperial Japanese Navy Academy in 1914. He served his midshipman duty on the cruiser and battleship. Commissioned a sub-lieutenant in 1915, he was assigned to the cruiser. After completing torpedo and gunnery schools, he served on the crew of the cruiser and destroyers, and. After graduation from the Navy Staff College in 1926 and his promotion to lieutenant commander, he was assigned his first command: the destroyer. He served as an instructor from 1929–1938, specializing in destroyer operations and torpedo warfare, before returning to sea again as the commander of Iwate. In October 1940, he became commander of the cruiser. At the time of the attack on Pearl Harbor, Koyanagi was captain of the battleship. He was promoted to rear admiral in December 1942. Later, Koyanagi commanded Destroyer Squadron 10 during the Guadalcanal Campaign. He was appointed Chief of Staff of the IJN 2nd Fleet under Admiral Takeo Kurita in July 1943. A strong believer in the Kantai kessen naval doctrine of a decisive battle, Koyanagi criticized the battle plans for the Battle of Leyte Gulf in October 1944, as the plan called for concentrating Japanese attacks on enemy transports rather than warships. During the Leyte campaign, he was aboard the flagshipAtago with Kurita when the fleet was ambushed by the American submarines and. Atago was hit by four torpedoes from Darter and capsized at 05:53 on 23 October 1944. Koyanagi survived and transferred to the battleship with Kurita. He was later severely injured by fragments from a near miss on 26 October and hospitalized until August 1945. Surviving the war, Koyanagi was promoted to vice admiral on 1 November 1945, and retired 10 days later. After the war, Koyagani was interrogated by Lieutenant Commander James A. Field, Jr. of the United States Navy Reserve in Tokyo on 24 October 1945. Field noted that Koyanagi clarified questions that the U.S. Navy strategists had on the Japanese decision to go north rather than enter Leyte Gulf on the morning of25 October. Koyanagi died on 6 August 1978.
