[3][4] While a student at the University at Buffalo, Hallion got his first umpiring job through his friend Jack Keeley; he worked games for the Saugerties Athletic Association softball league.

He was among the umpires who lost their jobs after resigning as part of a failed union bargaining strategy in 1999, but he was rehired in a December 2004 settlement with MLB.

[7] Hallion is known for his exciting and demonstrative strikeout signal in which he twists his body 180 degrees, which has earned him the nickname "Tornado Tom".

[9] Hallion was the home plate umpire on July 12, 1997, when Francisco Córdova and Ricardo Rincón of the Pittsburgh Pirates combined on a 3–0, 10-inning no-hitter against the Houston Astros.

He worked the 2006 World Baseball Classic and was the home plate umpire for the championship game between Japan and Cuba.

His crew was also selected to umpire the 2012 MLB Japan Series between the Seattle Mariners and Oakland Athletics.

A leaked recording from May 28, 2016, showed Hallion's heated argument with New York Mets manager Terry Collins after pitcher Noah Syndergaard was ejected for attempting to throw a beanball at Chase Utley, presumably as a payback attempt for Utley's slide in the previous year's NLDS which injured Mets shortstop Rubén Tejada.