William Henry Clapp was born in Montréal, Québec in 1879 to American parents. The family returned to the United States in 1885 and settled in Oakland, California. William H. Clapp came back to Canada in 1900 and enrolled at the Art Association of Montréal where he studied under William Brymner.
During this time he painted with Clarence Gagnon in St. Joachim and Baie St. Paul. He left Canada in 1904 to study in Paris at the Académie Julian. Clapp also took classes at the Académie de la Grande Chaumière and the Académie Colarossi. He was taken by the Impressionism and Post-Impressionism he found in France and adopted some of the techniques he witnessed. While in Europe he travelled and painted in Belgium and Spain.
In 1908, Clapp returned to Montréal where he continued to paint, teach, and exhibited his works. He showed paintings at the Art Association of Montréal's Spring Shows from 1908 to 1915 and again in 1918. He also took part in the Royal Canadian Academy's annual show in 1904 and again from 1908 to 1916 and in 1918. Clapp was made an Associate of the Royal Canadian Academy in 1911 and became a member of the Canadian Art Club in 1913, as well as a member of the Arts Club of Montréal and the Montreal Pen and Pencil Club.
Clapp left Montréal in 1915 and went to Cuba, where he stayed and painted until 1917. He then returned to Oakland and became curator of the Oakland Art Gallery (now the Oakland Museum) and was named director in 1920, he held this position until 1949.
In 1923, Clapp became a member of a well known group of California painters called the "Society of Six" and showed his paintings with the group until 1928. He also took up membership at the California Art Club, the Oakland Art League, the San Francisco Art Association and the Western Association of Art Museum Directors for which he became president in 1928. For six years he gave art classes in Oakland at the school he operated, the Clapp School of Art.
William Clapp's works can be found in various museum collections: National Gallery of Canada; Montreal Museum of Fine Arts; Musée national des beaux-arts du Québec; Oakland Museum; Oklahoma City Museum of Art; Smithsonian American Art Museum; Utah Museum of Fine Arts; Art Gallery of Ontario; Art Gallery of Hamilton; Art Gallery of Windsor.
Clapp died in Oakland, California in 1954 at age 75.