Stull was born in Hamilton, Ontario, Canada, in 1851. The son of a coach and hansom cab driver, he sketched people and animals throughout his childhood; he never had any formal training in art. His career of choice as a young man was that of professional actor. He joined a troupe in Toronto, Ontario, but did not meet with success; instead, he went to work for a Toronto insurance firm. By 1872 he was in New York City, where he hoped to find more opportunities as an actor. Instead, he again worked as an insurance clerk, involving himself with an amateur theatre group at the same time. It was his employers in the insurance business who first noticed his talents at drawing and set him to work illustrating for insurance files. They also encouraged him to show his portfolio to various periodicals in New York City. He found employment as a staff illustrator for Leslie's Illustrated Weekly, where he first produced caricatures and cartoons. His work was also published in other newspapers and magazines.
Stull began making his living as a racehorse portraitist in 1876 when he drew the winning horse in the Withers Stakes at Jerome Park, a racecourse in New York City. The drawing was published in The Sporting New Yorker. The horse's owner, August Belmont, saw the drawing and brought it to the attention of Elisha Buck, the editor of The Spirit of the Times, a sporting periodical; as a result, Stull worked there as an illustrator for fourteen years. In 1877 he began painting racehorse portraits on commission, staying at farms where his subjects were stabled to make his preliminary studies and finishing the paintings in his studio in New York City. He did not meet with immediate success when he began painting in earnest in 1879. He received enough commissions that he was able to cut back on his frenetic illustrating career, but although he made considerable efforts to improve his painting, he lacked technical skill. After receiving a great deal of harsh criticism he vanished from the equine world in 1884. Six months later he reappeared, having acquired markedly better skills at depicting the horse; it is believed that he spent at least part of his absence studying equine anatomy at a veterinary school.
Stull's work continued to improve throughout his career. In 1886 he, like many of his contemporaries such as J. M. Tracy (qv), began incorporating the findings of Eadweard Muybridge, an English photographer whose kinematical studies of animals and humans led to the first accurate depictions of a galloping horse, into his paintings. Initial reception of this new style varied from cool to scornful, but by the late 1890s he was established as one of the top horse portraitists in the country. He gave up illustration altogether and made an arrangement with the Coney Island Jockey Club in New York City to paint works for their clubhouse. He continued to paint for the top thoroughbred breeders and owners. In 1899 he went to England and France, painting for patrons there, including the Prince of Wales, for whom he painted the future English Triple Crown winner Diamond Jubilee, both as a foal at the foot of his dam, Perdita II, and later as a two-year-old. Stull's work appears occasionally in colored lithographs, sometimes printed by Armstrong of Boston, including the equine portraits inscribed Nevada, Ten Broeck and Thora, although there is no biographical information to indicate whether these prints were authorized by the artist or struck posthumously. Besides equine subjects, he occasionally painted sporting dog breeds including setters, poodles, and fox terriers. He was particularly active in the period from 1900 to 1910, during which decade he executed nearly half of his lifetime output of oil paintings, making more journeys overseas in addition to his trips to the thoroughbred farms of New York, Tennessee, and Kentucky. His health deteriorated badly in 1910 and records show he painted only two commissions in 1911.
His work is in many private collections, as well as at pari-mutual racetracks in the United States. The Haggin Museum in Stockton, CA, has Stull's Jockey on a Horse. The National Museum of Racing and Hall of Fame in Saratoga Springs, NY, has numerous works, including Hanover, Iroquois, The 1887 Brooklyn Handicap; the Clark Institute in Williamstown, MA, has his Kingston. Other institutions holding his work include The New York Jockey Club with offices in New York City and Lexington, KY, and the National Sporting Library in Middleburg, VA.
Stull died in New York City on 18 March 1913.