In 1971, he was hired on a full-time basis as the linebackers coach. He later joined the coaching staffs at
Southern Methodist University and the
University of North Carolina at Chapel Hill under the same position. In 1978, Darnell became assistant head coach and defensive coordinator at
Kansas State University. Darnell and the new staff turned around a program that went 0–11 in 1977, taking them to the
Independence Bowl in 1982, Kansas State's first bowl appearance. In 1986, Darnell joined the staff of
Al Groh at
Wake Forest University as assistant head coach and defensive coordinator. He spent two years with the
Demon Deacons before accepting the same position at the
University of Florida in 1988. In each of his two seasons at Florida, his defenses ranked third nationally and first in the
Southeastern Conference. In the middle of the 1989 season, he took as interim head coach after
Galen Hall was forced to resign, leading the Gators to a 3–4 record over the season's final seven games. In 1990, he accepted a position under
Lou Holtz at the
University of Notre Dame as assistant head coach and defensive coordinator. Darnell was replacing
Barry Alvarez, who left to take the head coaching job at the
University of Wisconsin–Madison. He later joined
John Mackovic's staff at the
University of Texas at Austin as assistant head coach and special teams coordinator in 1992. Two years later, he moved from special teams to defensive coordinator, serving in that position through the 1996 season. He then moved on to become Western Michigan University's head coach in 1997.
Texas A&M Darnell was hired as the defensive coordinator at
Texas A&M University by head coach
Dennis Franchione for the 2006 season. Franchione had previously served as Darnell's offensive coordinator at Tennessee Tech in 1983 and 1984. Inheriting a team that finished 107th in total defense and 97th in scoring defense, Darnell implemented a quick turnaround, with the 2006 team finishing 37th in total defense and 32nd in scoring defense. A day after head coach
Dennis Franchione resigned, A&M athletic director
Bill Byrne named Darnell the interim head coach. Darnell coached the Aggies in the 2007
Alamo Bowl, which was Darnell's 12th bowl to coach. ==Head coaching career==