McMahon was an assistant coach at Utah State for the last eight seasons and spent 11 seasons with the Aggie program. His eight-year tenure as an assistant at Utah State was the fourth-longest in school history.
McMahon replaces Tony Levine, who left last week to take a job as an assistant special teams coach with the Carolina Panthers. McMahon coached the defensive line at Utah State for the last five seasons and also served as the special teams coordinator from 1998-2000. He also spent five seasons as recruiting coordinator.
"We're certainly going to miss Tony Levine, who spent three productive seasons at the University of Louisville," said Petrino. "We're excited about bringing Tom into our program. He's a dedicated and bright young coach. He'll bring a lot of enthusiasm and energy to our staff."
McMahon, 36, joined the Utah State staff under John L. Smith in 1995, serving three years as a graduate assistant with the defensive ends. One of his players, Ben Crosland, earned Big West defensive player of the year honors in 1997.
McMahon was named a fulltime assistant coach in 1998 and spent the next three seasons working with the Aggie linebackers. During that time he coached first-team All-League selections Brent Passey, Blake Eagal, Jesse Busta and Big West Defensive Player of the Year Tony D'Amato.
A native of Helena, Mont., McMahon began his coaching career at his alma mater of Carroll College. After concluding his four-year playing career, he continued as the school's secondary in 1992 and 1994. In between he spent one season at Bozeman (Mont.) High School.
McMahon is married to the former Kim Sullivan and they have three children, Dawson, Emmett and Quincy. McMahon earned his bachelor's degree in math and secondary coach education at Carroll in 1992 and added a master's degree in secondary education from Utah State in 1997.
In another staff change, defensive line coach Kevin Wolthausen was promoted to co-defensive coordinator.