The move reunites Gilmore with defensive coordinator Vic Koenning, who served in the same capacity at Illinois from 2010-11 and coached the team to a victory as interim head coach in the 2011 Kraft Fight Hunger Bowl.
"Coach Gilmore is a great coach that understands his players to the fullest and is able to bring out their full potential," Mercilus said Saturday. "He is one of the rare coaches who can reach the players he coaches in a special way. I loved him as a person and coach."
In 2011, Gilmore's unit broke the school record with 41 sacks and ranked fourth nationally in tackles for loss (nearly eight per game).
Gilmore was the lone holdover on staff when Tim Beckman was hired as Ron Zook's replacement following the 2011 season.
The Illini have seen a staff shakeup since finishing 2012 with a 2-12 record. Gilmore joins offensive coaches Chris Beatty (coordinator, quarterbacks) and Luke Butkus (offensive line) in departing from Illinois.