Hadnot also played for the Cleveland Browns, Arizona Cardinals, and San Diego Chargers.

Hadnot started on the offensive line during his junior and senior years, and was named to the Second-Team All-District squad as a junior in 1997 and a First-Team All-District 12-5A selection in his senior year, when he helped lead his team to a 10–4 overall record and reached the quarterfinals of the state playoffs.

Hadnot played in all 11 games as a redshirt freshman, spending the season as a backup to Nathan Woody and Darnerius Watson at the guard position.

He was named Houston Offensive Player of the Week versus Texas Christian and Southern Miss.

He made the first start of his NFL career at left guard in place of the injured Jeno James at Seattle on November 21.

Inconsistency at both guard positions as well as left tackle for most of the season had a negative effect on Hadnot's performance.

Following the 2007 season Hadnot received an extra $231,269 from the NFL's "Performance Based Pay" system, which is given by the league for players who outperform the value of their current contracts.

It was announced on May 25, 2007, by Head Coach Cam Cameron that Rex Hadnot will switch back to right guard, leaving the center job up for grabs.

Rex is a cousin of former Texas Tech running back James Hadnot, a third-round draft choice of the Kansas City Chiefs in 1980 who played with the Chiefs through 1983, and also a cousin of former NBA-ABA basketball player Jim Hadnot.

He is also a cousin of Jermichael Finley, a former Texas tight end and third-round draft pick of the Green Bay Packers in 2008.