Peter Moran was the brother of Edward and Thomas Moran, the youngest of the three sons. He was apprenticed for a short time with Herline and Hersel, a lithographic printing firm, before leaving to study with his brothers in their Philadelphia studio. Peter showed an interest in painting and etching animals, a subject different from his two brothers. Peter became highly interested in Indian ethnology which motivated his trips out West, the first being as early as 1864. He returned in 1881 with Captain John G. Bourke to study the Pueblo Indians and again in 1890 to prepare an Indian census.