After completing torpedo and gunnery schools, he served on the crew of the cruiser Iwate and destroyers Kisaragi, Maki and Kuretake.

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.

At the time of the attack on Pearl Harbor in December 1941, Koyanagi was captain of the battleship Kongō, and he commanded her during the Battle of Midway in June 1942.

[3] During the Leyte campaign, he was aboard the flagship Atago with Kurita when the fleet was ambushed by the American submarines USS Darter and Dace.

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 of 25 October.