Mullen was a well-known comic illustrator by the time he became a “special artist” for Frank Leslie’s Illustrated Newspaper in 1864. He provided humorous illustrations for Vanity Fair from 1859 to 1863 and illustrated such humor books as Drifting About or What Jeems Pipes of Pipesville Saw and Did; An Autobiography by Stephen Massett (New York, 1863) and The Life and Adventures, Songs, Services, and Speeches of Private Miles O’Reilly by Charles Halpine (New York, 1864). In 1864, Frank Leslie recruited Mullen to join the war effort and sent him to draw the siege of Petersburg. He arrived with Andrew McCullum, another new Leslie’s artist, in late July just in time to witness the explosion of a mine placed in a tunnel under the Confederate lines. In spite of the explosion, the Union forces suffered terrible losses in their attack. Mullen recorded the extent of the carnage in often excruciating and grisly detail. In the following months, he continued to focus on the details of battle and its aftermath, particularly burial squads at Antietam and Petersburg, to send back images that captured the difficulties the Union forces were facing. By 1865, Mullen was back in New York and contributing illustrations for Artimus Ward: His Travels by Charles Farrar (New York, 1865), a humorous tribute to the editor of Vanity Fair. Along with other veteran “special artists”, Mullen’s drawings appeared in Beyond the Mississippi (Hartford, 1869); and in the aftermath of the Chicago fire, he did illustrations for Mrs. Leary’s Cow: A Legend of Chicago by C.C. Hine (New York, 1872). The Becker Collection contains drawings done by Mullen in Virginia and Washington, D.C. in 1864.