Boston Custer was the youngest brother of General George Custer and two-time Medal of Honor recipient Captain Thomas Custer. He was killed at the Battle of the Little Bighorn, on June 25, 1876, along with these two brothers. Boston Custer was born in New Rumley, Ohio, on October 31, 1848, one of five children born to Emanuel Henry Custer and Maria Ward Fitzpatrick Custer.

Boston Custer had been unable to join the Army due to poor health. As a civilian contractor, he served as forage master for General Custer's 7th Cavalry in the 1874 Black Hills expedition. He was employed as a guide, forager, packer and scout for the regiment for the 1876 expedition against the Lakota Indians.

On June 25, 1876, Boston Custer was with the pack train at the rear of the column. Boston overheard from a messenger that General Custer had requested ammunition and Captain Frederick Benteen’s Troopers for an impending large fight, and quickly left the pack train. Riding a mule, Boston passed by Frederick Benteen's Troopers and joined Custer's main column as it moved into position to attack the Indian camp. Boston’s trip on a pack animal took fifteen minutes, and fuels speculation about whether Captain Benteen could have came to General Custer’s aid. If Boston had stayed where he was assigned with the pack train, he would have surely survived the battle.

Boston was killed on Last Stand Hill. Like the others on the battlefield, a marble marker rests at the approximate place where his body was found. Though originally buried on the battlefield, Boston Custer's remains were exhumed, one of only two exceptions to the rule that only commissioned officers would be shipped home for reburial. He was reburied January 8, 1878, at Woodland Cemetery in Monroe, Michigan, near today's Monroe County, Michigan Museum.