William Harpin Mellor (June 6, 1874 – November 5, 1940) was a Major League Baseball first baseman. Mellor played for the Baltimore Orioles in . In 10 career games, he had 13 hits in 36 at-bats, with 5 RBIs. He batted and threw right-handed. Mellor attended both Brown University and the University of Virginia.
Mellor was born in Camden, New Jersey and died in Bridgeton, Rhode Island. He served as the head baseball coach at Connecticut Agricultural College—now known as the University of Connecticut—for one season, in 1920.