Lett played college football at Emporia State University and was selected in the seventh round of the 1991 NFL draft by the Cowboys, where he spent 10 seasons.
He received a scholarship from Auburn University, but after a low score on the ACT, he had to play two seasons at Hinds Community College in Raymond, Mississippi, compiling 141 tackles, 10 sacks, nine pass deflections and three fumble recoveries.
[2] As a junior in 1989, he helped the team win the Central States Intercollegiate Conference (CSIC) title and reach their first NAIA Football National Championship game.
In his second season, he blossomed into a key player in the Cowboys' defensive line rotation and his teammates nicknamed him "Big Cat" in reference to his agility.
However, Lett prematurely began to celebrate and the ball was knocked out of his hand by Bills receiver Don Beebe which resulted in a touchback.
In the divisional playoff game against the Green Bay Packers, he registered 6 tackles (1 for loss), 2 passes deflected (one was intercepted), 3 quarterback pressures and recovered a fumble.
In Super Bowl XXVIII, besides having 6 tackles, he forced Thurman Thomas to fumble in the third quarter, leading to a James Washington 46 yards return for a touchdown, which tied the score at 13–13 and changed the momentum of the game.
In 1994, he started all 16 contests and was named to his first Pro Bowl after finishing with 68 tackles, 4 sacks and 26 quarterback pressures (second on the team).
Lett would be suspended at different times for violating the NFL substance abuse policy, missing a total of 28 games during his career.
His last season in Dallas was 2000, when he played in 9 games (7 starts), registered 35 tackles (14 solo), 2.5 sacks, 4 passes deflected and one forced fumble.
He sprained his left medial collateral ligament during a November practice and was inactive for five games, before being placed on the injured reserve list on December 14.
In the resulting chase for possession, the Dolphins recovered in the end zone, but due to muffed kick rules, the ball was awarded to them on the one-yard line.
Since Lett went for the ball and touched it, by rule the play was considered a muffed kick and the Dolphins were given possession on the one-yard line with three seconds showing on the clock.
[12] The most meaningful impact of Lett's miscue, assuming all remaining results were the same, was that it ensured the team's last game of the season had playoff implications.
Ultimately, the play did not derail the Cowboys' title defense, as they won all of their remaining games after Thanksgiving and went on to win Super Bowl XXVIII.