Born in Philadelphia, by the 1890s he was based in Chicago, where he met Baum. Besides the best known of all, The Wonderful Wizard of Oz, books written by Baum and illustrated by Denslow (copyrights generally being held jointly) included By the Candelabra's Glare, Father Goose: His Book, and Dot and Tot in Merryland.
After Denslow quarreled with Baum over royalty shares from the 1902 stage adaptation of The Wizard of Oz, for which Baum wrote the script and Denslow designed the sets and costumes, Baum determined not to work with him again.
His career declined thereafter, and he died in obscurity.