Timothy Henry O'Sullivan (c. 1840 – January 14, 1882) was a photographer widely known for his work related to the American Civil War and the Western United States.
In 1861 and 1862 O’Sullivan photographed the Civil War for Mathew Brady, but spent the rest of the war working for Alexander Gardner. His war photographs were published in Photographic Incidents of the War from the Gallery of Alexander Gardner, Photographer to the Army of the Potomac and Gardner’s Photographic Sketch Book of the War, 1865/1866. In 1867 he was appointed to Clarence King’s Geological Explorations of the Fortieth Parallel and photographed for King in 1867-1869 and again in 1872 in California, Nevada, Utah, Wyoming, Colorado, and Idaho. O’Sullivan spent six months of 1870 with the Darien Expedition, photographing in present-day Panama, but the wet weather and heavy foliage hampered much successful work. That same year he was hired by Lieutenant George Wheeler to participate in his explorations, eventually known as the United States Geographical Surveys West of the One Hundredth Meridian. Between 1871 and 1874, O’Sullivan spent three seasons photographing for Wheeler in California, Nevada, Arizona, Colorado, Utah, and New Mexico. During his time with the western surveys, most winters were spent printing negatives made during the explora- tion season. 1874 marked his last year photographing in the West, after which he returned to Washington D.C., where he continued to work, including a brief job with the United States Treasury Department in 1880–1881. He left the government position just five months after beginning, due to tuberculosis, from which he died on 14 January 1882, at age 42 .