Taken to New York City at age seven, Harry Watrous was educated in several private schools in Manhattan. He received some training in art from his tutor, F. Hatch, and was enrolled for one term in the National Academy's Antique School in 1873. After turning down an appointment to the U.S. Naval Academy, he began work as an artist, painting bucolic landscapes. In 1881 he accompanied Humphrey Moore to Europe, passing through Paris to spend nine months in Spain and North Africa. Returning to Paris the following year, he shared a studio with George R. Barse and studied for two terms with L‚on Bonnat and an additional season at the Académie Julian. By 1884, Watrous was working on his own, seeking criticism from Jean Jacques Henner, Felix Ziem, and Charles E. Frère. He adopted the minute technique of Jean Louis Ernest Meissonier, producing cabinet-sized scenes of pre-revolutionary France. Apparently of a belligerent nature, he often found himself involved in altercations with Europeans and North Africans, including one duel for which John S. Sargent served as his second.
Back in New York in 1886, Watrous married artist and novelist Elizabeth S. Nichols. They built a summer home on Lake George in 1891. When his eyesight began to fail at the turn of the century, Watrous abandoned his meticulous Meissonier-inspired manner and began to paint larger, broader female studies and landscapes. In the latter category, he was influenced by Ralph Blakelock, a good friend whose work Watrous championed, sold, and following Blakelock's death, authenticated. The final phase of his career began in 1923, when he developed a nearly exclusive devotion to still life painting.
Watrous was extremely active in Academy affairs, serving on the Council (1897-8), as recording and corresponding secretary (1898-1920), as vice-president (1920-33), and as president. He won several awards in Academy Annuals; in his will he left $5000 to establish a fund for another prize, the Elizabeth Watrous Gold Medal.