In 2001, Nix became the defensive coordinator at Southern Miss at the age of 29, making him the youngest coordinator in Division I-A. By 2003 he was a finalist for the Broyles Award, which recognizes the nation’s top assistant coaches. His impact as a defensive coordinator was immediately felt as he led the 2001 Golden Eagle defense, which had lost 7 starters from the previous season, to rankings of 8th in the nation in scoring defense and 11th in the nation in total defense. In his first three years as defensive coordinator at USM, Coach Nix’s defenses stayed in the top 15 nationally in scoring defense.
South Carolina
Coach Nix began his tenure at South Carolina in 2005 as co-defensive coordinator, sharing the responsibility with his long-time colleague John Thompson. Near the end of that season, impressed by Coach Nix’s coaching prowess, Head Coach Steve Spurrier promoted him to sole defensive coordinator and assistant head coach. Prior to his promotion, the Gamecocks were giving up over 27 points per game. After the promotion, and under Coach Nix’s sole guidance, the USC defense held opponents to just 17 points per game and had wins over the University of Florida and the University of Tennessee which hadn't happened in over 20 years.
Ole Miss
On December 11, 2007, Nix accepted the job as defensive coordinator at the University of Mississippi, to serve under new Ole Miss head coach Houston Nutt. During the 2008 season, Nix had Ole Miss defense ranked among the national leaders in points per game yards per game and ranked 5th nationally in total rushing yards The 2008 defense included firth team All-American DT Peria Jerry and third team DE Greg Hardy. The season was capped off with a 47-34 win over Texas Tech in the Cotton Bowl and an 8-4 record. Following the 2009 season, Houston Nutt promoted Nix to the role of assistant head coach after being nominated for the Frank Broyles Award. Upon the hiring of Hugh Freeze as head coach of the Ole Miss Rebels on December 6, 2011, after Nutt was fired, Nix was dismissed as well.
After interviewing with the Oklahoma Sooners, Nix was named co-defensive coordinator at Middle Tennessee State on February 22, 2012 under Rick Stockstill. Nix improved the defense tremendously in his first year on the job. He was fired December 28, 2016.
Texas A&M
Nix served as the senior defensive analyst for the Aggies in the 2017 season.
UTSA
On Feb. 3, 2020, Nix was named defensive coordinator and linebackers coach for the UTSA Roadrunners.