Kirn graduated from the United States Naval Academy in 1932 and became an aviator. During World War II he served in the Solomon Islands campaign, including the Battle of Edson's Ridge, during which time he was awarded the Navy Cross and the Distinguished Flying Cross. Later in his career he was awarded the Navy Distinguished Service Medal for his service as United States Naval Attaché and United States Naval Attaché for Air at the U.S. Embassy, London.