Probably due to his experience on the fourth Fisk expedition, Illingworth was selected as photographer to Custer's 1874 military expedition by then-Captain William Ludlow (referred to as "Col. Ludlow" by journalists covering the expedition, perhaps because he received a brevet during the Civil War). Ludlow was the Chief Engineer of the Department of Dakota, and was in charge of mapping and scientific data collection for the expedition. He provided Illingworth necessary equipment, rations and supplies, and added Illingworth to the civilian payroll as a "teamster", with a salary of $30 per month. In return, Illingworth was expected to provide six sets of his exposed plates to the US Army. The wagon carrying Illingworth's equipment is estimated to have weighed nearly 400 lbs (c. 182 kg). Illingworth and the U.S. 7th Cavalry Regiment departed for the Black Hills on 2 July 1874 from Fort Abraham Lincoln on the west bank of the Missouri River, seven miles south of what is now Mandan, North Dakota, and returned 30 August 1874.