Hays was born in New York City, and studied drawing there under John Rubens Smith (1775-1849). He achieved his first and greatest success as a painter of animals. His western paintings "The Wounded Buffalo" (1864 or 1865, American Museum of Natural History), "The Stampede" (1862, American Museum of Natural History), and "The Herd on the Move" (1862, Gilcrease Institute) helped establish his reputation as one of America’s greatest animal painters of the mid-nineteenth century.
Hays’ interest in still life evolved over the course of the late 1850s and early 1860s. In 1857 he exhibited a still life of strawberries at the National Academy, and five years later showed a picture of fruit and nuts at the Pennsylvania Academy of the Fine Arts. In the mid-1860s he began to paint sketches of orchids and wildflowers. Apparently, Hays had a small conservatory at his house in Manhattan where he grew his own plants.
Upon Hays’ death in 1875 a writer for the New York Evening Post remarked that “his pictures were always marked by careful study and never showed hurried treatment. He did not, confine his attention entirely to animal painting, but sometimes sent out from his studio a flower and other studies of the most brilliant character” (“Obituary. William J. Hays,” New York Evening Post, March 15, 1875, p. 4).