He played on the Aggies' 1967 squad, which garnered a Southwest Conference championship and a Cotton Bowl Classic win.
In 1987, Hallman became the head coach at Southern Miss, where he proceeded to compile the highest winning percentage in school history with the help of future NFL star quarterback Brett Favre.
Over the next two seasons, Southern Miss gained a reputation as "giant killers," thanks to victories over national powers such as Florida State and Alabama, all on the road.
He is credited for laying the foundation for Southern Miss' rise to regional power status under Jeff Bower.
During Hallman's first season in 1991, several of Hallman's football players were accused of instigating a fight with LSU men's basketball players, including All-American Shaquille O'Neal, in Broussard Hall, LSU's athletic dormitory, two days prior to the Tigers' contest with Mississippi State.
LSU started the 1991 season with one-sided losses to Georgia (31–10) and Hallman's alma mater, Texas A&M (45–7), and finished 5–6.
LSU led 23–9 early in the fourth quarter, and the Bayou Bengals were in good position to end Auburn's 13-game winning streak.
But LSU quarterback Jamie Howard threw two interceptions that were returned for Auburn touchdowns, tying the game.
Hallman continued working at the college level as a position coach and coordinator for another decade, with stops at SEC schools Alabama and Mississippi State.