Diaco played college football at the University of Iowa, under head coach Hayden Fry, who described Diaco as "extremely tough, very intelligent". He was a two-time All-Big Ten selection as a linebacker and was named the team’s co-MVP in 1995, starting in all 23 games over his junior and senior seasons. Diaco was teammates with Mike Devlin, Scott Slutzker, Danan Hughes, and Mike Wells.
Coaching career
Diaco began his coaching career in 1996 as a graduate assistant at Iowa. From 2001 to 2004, Diaco was an assistant coach at Eastern Michigan, coaching running backs, linebackers, and special teams. He became the linebackers and special teams coach at Virginia in 2006, coaching under Al Groh.
Cincinnati
Diaco served as the defensive coordinator at Cincinnati in 2009 under Brian Kelly. Cincinnati finished the year ranked 44th in NCAA Division I-A in points allowed per game and 23rd in sacks.
Notre Dame
Diaco followed Kelly to Notre Dame in 2010, taking the same position as defensive coordinator. In 2012, Notre Dame finished the year ranked second in points allowed per game and appeared in the 2013 BCS National Championship Game. Diaco was awarded the Broyles Award, which recognizes the top assistant coach in college football.
Connecticut
In December 2013, Diaco became the head coach at the University of Connecticut, succeeding interim coach T. J. Weist, who replaced Paul Pasqualoni. Diaco was replaced at Notre Dame by Brian VanGorder. While at Connecticut, Diaco coined the term "Civil Conflict" to describe games between the Connecticut Huskies and the UCF Knights. This one-way declaration of a "rivalry" was first derided and then ignored by UCF. In 2015, Diaco's Huskies beat undefeated Houston 20–17 at Rentschler Field to make the Huskies bowl-eligible for the first time since Edsall left. At this point many UConn fans believed that Diaco was the future of the program after taking a 2–10 team to a bowl game the next year. After an exciting 2015, the Huskies were projected to be bowl-eligible in 2016, but hope quickly faded after a weak win over FCS Maine, and a botched goal-to-go situation vs. Navy that would have put the Huskies at 2–0. The Huskies traveled to Houston at 2–2, but got blown out. UConn then went on to beat Cincinnati 20–9, but then lost out to finish 3–9. Connecticut fired Diaco on December 26, 2016, effective January 2, 2017. His buyout, renegotiated after the 2015 season, was $3.4 million. Media reports indicated that Diaco's refusal to consider former Minnesota head coach Jerry Kill for the position of offensive coordinator may have led to his termination. Connecticut hired former head coach Randy Edsall to replace him. Diaco finished his head coaching career at the University of Connecticut having the worst winning percentage amongst Huskies' head coaches since 1920.
Nebraska
In 2017, head coach Mike Riley hired Diaco to serve as the defensive coordinator at Nebraska. He was fired along with Riley and most other assistant coaches on November 25, 2017 after the Cornhuskers finished 4-8.
