Born in Rome, then still Lazio and not part of Italy as such, the son of the wealthy American landscape painter William Stanley Haseltine (1835–1900) who was associated with the Hudson River School and Luminism, he studied at Harvard University. After graduating in 1899, Haseltine went to Munich Academy in Germany to study drawing and then to the Académie Julian in Paris, France where he studied painting.
After his first piece of sculpture met with success, he pursued that artistic avenue. Inspired by the gathering of artists from around the world to the Montparnasse Quarter of Paris, Haseltine chose to make Paris his home for the next thirty-five years until the German occupation of France during World War II. He moved to the United States where he remained until 1947 at which time he returned to France.
Equestrian statue of Sir John Dill statue at Arlington National Cemetery by Haseltine.
In 1940 he was elected into the National Academy of Design as an Associate member and became a full Academician in 1946.
Haseltine sculpted a variety of animals but is best known for his equestrian sculptures, most notably the 1934 life-size statue of the thoroughbred race horse Man o’ War at the Kentucky Horse Park in Lexington, Kentucky and “George Washington on Horseback”, Gilded bronze statue at the Washington National Cathedral made in 1959. He also traveled to India, where he made an oversized statue of one of the ancestors of the Maharaja of Nawanagar, Jam Shri Rawalji in 1933. It can still be seen there. He replicated many of his large works in table-top sizes. The author of a number of books on animalier art, Haseltine was well connected in American upper class society and did a three-year project to create a work for heiress Barbara Hutton. This project included two horses heads which were gilded bronze, with precious and semi precious stones. After her death the heads disappeared and resurfaced a few years ago at an auction in New York.