Thomas Mower McDougall subsequently he was appointed Captain, 15 Dec 1875. In 1876 Captain McDougall commanded Company B and accompanied the 7th Cavalry Regiment, under command of Lt. Col. George A. Custer in its fateful departure from Fort Abraham Lincoln to participate in what became known as the Great Sioux War of 1876, which culminated in the Battle of the Little Bighorn, also known as Custer's Last Stand which occurred on June 25–26, 1876. On May 17, 1876, the 7th Cavalry left their base at Fort Abraham Lincoln near present day Mandan, North Dakota to join in a campaign against the Sioux Indians.
Thomas Custer in the Yellowstone Expedition of 1873, where he fought in the Battle of Honsinger Bluff, and the Black Hills Expedition of 1874. Custer was appointed captain in 1875 and given command of Company C of the 7th Cavalry. In 1874, at the trading post at Standing Rock Agency, Custer participated in the arrest of chief Rain-in-the-Face (Lakota), suspected in the 1873 murder of Dr. John Honsinger. During the 1876 Little Bighorn campaign of the Black Hills War, he served as aide-de-camp to his older brother Lt. Col. George A. Custer and died with his brother on June 25, 1876. Lt.