Horn was the son of Dawson and Gracie Horn. He graduated from A. H. Parker High School in 1943. He taught at Washington Elementary School, Western-Olin High School and Parker High School, and also coached track and field. He retired from Birmingham City Schools as the system's Director of Middle School Athletics.
Horn, along with fellow legislator John Rogers, came to be called the "Sin Twins" for their tireless support for legalized gambling. None of their lottery of casino bills passed, but they did succeed in creating the Alabama Horsemen's Benevolent and Protective Association to make possible the development of the Birmingham Turf Club horse track.
In 1993 Horn promised his constituents that he would pass a bill to change the appointed Birmingham Board of Education to an independently elected body. He used his power as chair of the Finance and Taxation Committee to kill every Jefferson County bill that came before him.
In the 1994 election, Horn was successfully challenged by Rodger Smitherman, who had the backing of Richard Arrington's Jefferson County Citizens Coalition. Horn tried to win his seat back in 1998, 2002, 2006 and 2010, but was unsuccessful.
Horn and his wife, the former Mildred Douglas, had four children: William Jr, Karen, Darryl and Geri. He died after a long illness in December 2018. Services were held at St Paul Evangelical Lutheran Church and he was interred at Elmwood Cemetery.