Hoppin was born in Providence in 1807. He was the brother of James Mason Hoppin. His English ancestor, Thomas Hoppin, settled in Massachusetts about 1635. Descendants removed to Rhode Island before the Revolution, when Benjamin Hoppin proved his patriotism by resigning a colonelcy under the King to become a captain in the Continental Army. Benjamin Hoppin's son, another Benjamin, was a prosperous man of affairs in Providence. He and his wife, Esther Phillips Warner, who came from Middletown, Connecticut, had six children of whom William was the third.