William Mershon Lanning (January 1, 1849 – February 16, 1912) was an American lawyer, jurist, and Republican Party politician who represented New Jersey's 4th congressional district in the United States House of Representatives from 1903 to 1904. He later served as a United States district judge of the United States District Court for the District of New Jersey and as a United States Circuit Judge of the United States Court of Appeals for the Third Circuit and the United States Circuit Courts for the Third Circuit.