Civil War Union Brevet Brigadier General. After graduating from Harvard University in 1858, Hartwell became a tutor at Washington University in St. Louis, Missouri. When the Civil War began, he joined the 3rd Missouri Reserve Regiment and fought for the surrender of the pro-Confederate Camp Jackson. After relative peace was established in St. Louis in June of 1861, he returned to Massachusetts and began his studies at Harvard Law School. In September of 1862, he received a commission as a first lieutenant in the 44th Massachusetts Volunteer Infantry. He saw action in Goldsboro and Kingston, North Carolina. On January 26, 1863, Governor John Albion Andrew received permission to begin recruitment of African-Americans to man regiments of volunteer infantry. The "United States Colored Troops" were to be commanded by white officers. Initially, Hartwell was commissioned as senior captain of the 54th Massachusetts Volunteer Infantry. Because of excessive enlistments, a second regiment, the 55th, was formed and Hartwell was made its lieutenant colonel. On July 21, they embarked for New Bern, North Carolina. With the news of losses suffered by Robert Gould Shaw and the 54th Massachusetts Regiment, the 55th was ordered to Folly Island, South Carolina where they set up a base camp. The commanding officer, Norwood P. Hallowell was forced to resign in the fall of 1863 because of an injury; and Hartwell was elevated to colonelcy of the 55th. The issue of equal pay with white troops consumed much of Hartwell's time. He also advocated for the promotion of qualified African-American men to higher rank. On October 7, 1864, the pay issue was finally resolved to the satisfaction of the men. At the end of November of 1864, the 55th was sent on an expedition down the Broad River to destroy the railroad at Grahamville, South Carolina. On November 30th, they came upon a large Confederate earthwork fort hidden in the trees at Homey Hill. Hartwell led three separate charges in an attempt to take the fort. He was wounded three times and had his horse blown out from under him. He was removed from the field, treated, and sent home to recuperate. On December 30, 1864, he was breveted brigadier general "for gallant services at the Battle of Honey Hill." He rejoined his regiment in January of 1865 and served for the remainder of the war trying to keep central South Carolina secure. After the war, Hartwell returned to Massachusetts and practiced law. In 1868, he was offered and accepted the position of First Associate Justice of the Supreme Court of Hawaii by King Kamehameha V. In 1874, he was asked to serve as King Kalakaua's Attorney General. He reluctantly agreed, but resigned after one year to resume the practice of law. In Hawaii, he served on a number of boards and commissions. He served as editor of the Hawaiian Gazette, member of the Board of Trustees for the Planters' Labor and Supply Company, and president of the Pacific Cable Company. He supported the idea that the United States should acquire a permanent lease with Hawaii for a naval base at Pearl Harbor. After the overthrow of the Hawaiian monarchy in January of 1893, Hartwell served on the Annexation Commission. When Hawaii was annexed by the United States on July 7, 1898, he traveled to Washington to advise Secretary of State John Hay regarding Hawaii's future. On June 15, 1904, he was appointed Associate Chief Justice of the Supreme Court of Hawaii. He served in that capacity until August 15, 1907 when he was sworn in as Chief Justice. In February of 1911, he resigned and set sail for Europe. His vacation was cut short by illness and he returned to Hawaii. He died at his home in Honolulu on August 30, 1912. His grave is the westernmost grave of a Civil War general on American soil.