Timothy H. O'Sullivan and his family came to New York from Ireland soon after his birth. In his teens he apprenticed in the NYC studio of Mathew Brady. As early as 1856 he worked at Brady's Washington, DC, studio, which was managed by Alexander Gardner.
In May, 1861, when Brady travelled to Washington to photograph troop encampments, O'Sullivan was his assistant. That June, Gardner loaned him as an experienced field photographer to a visiting NY photographer. In July, he travelled with Brady and Alfred Waud, a magazine illustrator, to Bull Run to photograph the battle. From December, 1861, to May, 1862, O'Sullivan photographed military activities as well as spots of scenic interest in the South.
Upon his return to Washington he found that Gardner had left Brady and struck out on his own. He soon joined Gardner and worked as a civilian field photographer through the end of the war.
In the years 1867-1874, O'Sullivan worked as photographer to a number of federal surveys in the western and southwestern U.S., except for 1870 when he joined the Isthmus of Darien (Panama) Survey.
He contracted to print the Wheeler survey negatives, 1875-76. A partner in Wm. J. Armstrong's Washington firm of Armstrong & Co., 1878-79, he left when appointed photographer to the U.S. Treasury.