MacArthur was elected a member of the Military Order of the Loyal Legion of the United States (MOLLUS) in 1868 and was assigned insignia number 648. On May 6, 1908, he was elected commander of the Wisconsin Commandery of MOLLUS. He was elected as the Order's senior vice commander in chief on October 18, 1911, and became the Order's commander in chief upon the death of Rear Admiral George W. Melville on March 17, 1912. He held the office until his own death six months later.