 From 1870 to 1873, Illingworth worked as a photographer for railroad construction crews.Sometime in the 1870s, Illingworth and William McLeish formed a partnership called "Illingworth & McLeish", that produced stereographs.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).