Carollo started in the NFL as a side judge and officiated Super Bowl XXX in 1996 at that position, then became a referee (crew chief) for the start of the 1997 NFL season after Red Cashion and Howard Roe announced their retirements. Carollo was assigned to work his first post-season as referee during the 1998-99 NFL playoffs and later made his second appearance in the Super Bowl at Super Bowl XXXVII in 2003. Before he officiated Super Bowl XXXVII, Mike Pereira, the NFL's director of officiating, said of Carollo, "Bill is just perfect in the management of a game. He understands how to handle coaches, players and a team of seven officials. He's terrific at that." In addition, he was the alternate referee of Super Bowl XXXVIII.