On June 28, 2021, Harrison was appointed as the general manager and president of basketball operations for the
Dallas Mavericks, succeeding
Donnie Nelson. This appointment coincided with the hiring of
Jason Kidd as the head coach. During Harrison's first three seasons as GM, the Mavericks reached the
conference finals twice and the
NBA Finals in 2024. In June 2024 after reaching the NBA Finals, the Mavericks gave Harrison a multi-year contract extension.
Luka Dončić trade and firing Together with longtime friend
Rob Pelinka, Harrison engineered the
February 2025 trade of Mavericks superstar
Luka Dončić to the
Los Angeles Lakers, considered by many as one of the most shocking trades in the history of the NBA. The trade received overwhelmingly negative reactions from Mavericks fans and the media, especially given that the Mavericks had just made the NBA Finals the
previous season with a team built around Dončić. Public response to the trade included
death threats directed at Harrison, as well as "Fire Nico" chants, fan-made merchandise, and social media remarks for months after. The Mavericks organization subsequently ejected from the arena those chanting, holding up signs, and wearing shirts of "Fire Nico", leading to
increased protest. These chants were also heard at unrelated sports games, a
St. Patrick's Day parade, and a
Medieval Times dinner show. Additionally, fans and businesses installed billboards and murals calling for Harrison's firing. The continued public disapproval led to many calling Harrison "the most hated man in Dallas". Harrison defended his decision, stating "defense wins championships" and adding that
Anthony Davis exemplified the culture the Mavericks front office was trying to create. Davis subsequently reaggravated an abdominal injury during his first game with the Mavericks which kept him out for a significant period. Similarly, Harrison's mismanagement and training staff decisions were also criticized as a cause of the disproportionate
load management in the wake of the absence of Dončić on the court, leading to the season-ending injuries of Mavericks starters
Kyrie Irving,
Dereck Lively II, and
Daniel Gafford. Harrison also responded, "I did know that Luka was important to the fan base. I didn't quite know it to what level." Harrison's ignorance of Dončić's success, ability, and popularity drew harsh criticism from the majority of fans, including
Charles Barkley and other commentators. Harrison would end up using the first overall pick to select
Cooper Flagg, amidst continued chants of "Fire Nico". On November 11, 2025, Harrison was relieved of his duties with the Mavericks organization after a 3–8 start to the 2025–26 season. In an open letter to Mavericks fans, team owner
Patrick Dumont stressed that "this decision was critical to moving our franchise forward in a positive direction", alluding to the backlash directed towards Harrison and the team following the Dončić trade. ==Personal life==