After graduating from Yale, he enrolled at the Art Students League of New York where he took classes from William Merritt Chase (1849–1916), Kenyon Cox (1859–1919), Francis Edwin Elwell (1858–1922), and most important, Augustus Saint-Gaudens (1848–1907), who became his mentor.

After a short stint in Saint-Gaudens' private studio, Pratt traveled to Paris, where he trained with sculptors Henri-Michel-Antoine Chapu (1833–1891) and Alexandre Falguière (1831–1900) at the École des Beaux-Arts.

In 1892, he returned to the United States to create two large sculptural groups representing The Genius of Navigation for the World's Columbian Exposition in Chicago.

In 1893, he began a 25-year career as an influential teacher of modeling in the School of the Museum of Fine Arts, Boston.

(1) When Saint-Gaudens' uncompleted group for the entrance to the Boston Public Library was rejected, Pratt was awarded a commission for personifications of Art and Science.