A renowned graphic artist and prolific illustrator in the 1880s and 1890s, Brennan was particularly skilled in working with pen and ink. He was raised in Cincinnati and studied art at the University of Cincinnati School of Design, where he befriended fellow art students Kenyon Cox and Robert Blum. With his friends from Cincinnati, he moved to Philadelphia for a period of two years. He later worked with Frank Duveneck. Brennan settled in New York around 1879, where he illustrated for The Century, Life, St. Nicholas, and other magazines. Brennan also illustrated a number of children's books. His delicate pen-and-ink pictures translated perfectly into engravings, the dominant printing technology of the time.
One of many American artists inspired by the European etching revival initiated by James McNeill Whistler, Charles Meryon, Sir Francis Seymour Haden, and others. A primary catalyst of the etching revival in America was the journal American Art Review (1879–1881), founded and edited by Sylvester Rosa Koehler. Brennan was a contributing etcher, with Thomas and Peter Moran, Otto Bacher, J. M. Falconer, F. S. Church, Robert Swain Gifford, Henry Farrer, Samuel Coleman, and James D. Smillie. Because of production costs, the Journal lasted only slightly over two years.
