Tillinghast was active in the Rhode Island Militia and
National Guard. He was commissioned in the Rhode Island Militia as a 2nd lieutenant on June 18, 1896. He was promoted to captain on December 29 of the same year. On May 20, 1898, he volunteered for active duty as a captain and served as commander of Company A of the
1st Rhode Island Infantry during the
Spanish–American War. He was stationed with the regiment in Virginia, Pennsylvania and South Carolina but was not deployed overseas. He was mustered out of service, along with the regiment, on March 30, 1899, and returned to Rhode Island on April 1. In 1902 he became captain of the 2nd Company of the 1st Infantry Regiment of the Rhode Island Militia. He was elected as the lieutenant colonel for the Rhode Island Coast Artillery District on October 20, 1908. Tillinghast became colonel in command of the Coast Artillery District of Rhode Island on May 28, 1909. During
World War I, he was called into Federal service on July 25, 1917, and commanded
Fort Greble on Dutch Island in the West Passage of
Narragansett Bay in
Rhode Island. He was demobilized in December 1918 and retired from the Rhode Island National Guard on April 8, 1919. Upon his retirement, he was promoted to brigadier general with a date of rank of August 5, 1917. ==Yachting==