Photographer. Known for Civil War scenes and western landscapes, he overcame the harsh working conditions of wartime and rugged terrain to produce some of the nineteenth century's most technically and aesthetically compelling images. Born to Irish immigrant parents shortly before or possibly soon after they took up residence on Staten Island, he began his professional career as an apprentice in Mathew B. Brady's New York studio. Soon he moved to Washington, D.C., subsequently his home base, to work under Alexander Gardner in Brady's outpost. Upon the outbreak of the Civil War, he became a field reporter for Brady, but when Gardner established his own photographic enterprise, O'Sullivan went with him. Traveling for three years with the Union army, he was present at some of the war's most significant moments, including battles at Manassas, Petersburg, Antietam, Bull Run, Fredericksburg, Appomattox, and, importantly, Gettysburg. There O'Sullivan made a particularly memorable series, including one of his best-known images, “A Harvest of Death, Gettysburg, Pennsylvania,” which numbered among forty-five of his views that appeared in Gardner's 1866 Photographic Sketch Book of the War. In this poignant image, military corpses lie strewn in an abstract progression into the distance, as a distant lone horseman and another figure on foot bear witness. Although it presents a horrifying spectacle, with mangled bodies clearly visible in the foreground, the evocative, pale light and heavy July atmosphere soften harsh factuality, suggesting consolation and redemption. After the war, as attention turned to the nation's expansion, O'Sullivan turned his sights westward. Except for a relatively unproductive excursion to Panama with an 1870 survey, O'Sullivan spent most of his time from 1867 through the summer of 1874 roaming the West, usually in the company of federally sponsored expeditions. From time to time, generally during winter months, he returned to Washington to sort and print negatives. Convincingly conveying the drama and vastness of the region's natural features, he composed sharply focused landscapes with a modern eye for the large abstract masses of its grandeur. In one of the best known, the 1873 “Ancient Ruins in the Cañon de Chelle,” a forbidding cliff face soars beyond the top of the photograph, looming powerfully above remains of an ancient Indian pueblo. He also photographed contemporary Indians of the Southwest, creating dignified and unsentimental documents. During the late 1870s few opportunities opened, and he spent much of this time printing his backlog. In November 1880 he became chief photographer at the Treasury Department, but ill health forced him to resign after only five months. He died of tuberculosis at his father's home on Staten Island.