Nguyen attended Texas A&M University, where he played for the Texas A&M Aggies football team from 1995 to 1998. He battled the perception that he was too small to play linebacker for as long as he was involved in the game of football, despite his muscular 5'11", 238-pound frame. Proving critics wrong at an early stage, he finished his college career as the Aggies' career record holder with 51 consecutive starts, 517 career tackles and a 10.7 tackles-per-game average. His 517 career tackles is a Texas A&M record. He is the only Aggie to ever lead the school in tackles for four consecutive years. It is widely acknowledged that Nguyen was the undisputed on-field leader of the famous "Wrecking Crew" defense, as well as being its most famous and decorated player.
In 1995, he was named Southwest Conference Defensive Newcomer of the Year. From 1996 to 1998, he earned first-team All-Big 12 honors three consecutive times. In 1997, he was named Aggies MVP. In 1998, he was named a consensus All-American by every awarding publication, won the Bednarik Award for College Defensive Player of the Year and the Lombardi Award for Outstanding College Lineman. Nguyen was also selected as a finalist for the Butkus Award eventually becoming first runner-up. Nguyen missed winning the Butkus Award by a single vote out of more than 45 votes cast. It remains to date the closest vote in Butkus Award history. He was named Defensive Player of the Game for the 1998 Cotton Bowl after recording a bowl record 12 tackles, including 3 tackles for a loss, and an interception, which he lateraled to a teammate for a touchdown. Dallas Morning News named him Big 12 Male Athlete of the Year in 1998, and he earned Defensive Player of the Year honors in the Big 12 Conference. He also won the Jack Lambert Award in 1998. In 2005 he was named to the Big 12 Conference's 10th Anniversary Team. In 2010, he was voted on to the AP All-Time Big 12 Team tying with former University of Texas player Derrick Johnson for most votes at linebacker (19 out of 20.) Nguyen is generally acknowledged as the best defensive player in Texas A&M football history and one of the greatest defensive players of all-time in the history of the Big 12 Conference. In 2004, Nguyen was inducted in the Texas A&M Athletics Hall Of Fame. In April 2007, Nguyen was also inducted into the AT&T Cotton Bowl Hall Of Fame. Nguyen was most recently inducted into the Texas Sports Hall of Fame on February 27, 2014.
The Dallas Cowboys drafted Nguyen in the third round (85th overall) of the 1999 NFL Draft. Nguyen made an impact with the Cowboys from the very beginning, leading the team in special-teams stops as a rookie and becoming Dallas' starting middle linebacker in his second season. In 2001, he completed his first full season as a starting NFL middle linebacker and recorded 112 tackles. In 2003, with Nguyen leading the team in tackles, the Cowboys were the most effective defense in the NFL by a very wide margin; he was selected second-team All-Pro by the Associated Press.
Nguyen officially retired from the NFL on March 3, 2006 after a neck injury, which hindered his 2005 performance, failed to improve. He led the team in tackles three times, in 2001, 2003, and 2004, and amassed 516 tackles in seven seasons, despite missing half of two separate seasons to injury. As of November 2010, Dat Nguyen currently ranks 10th all-time in Cowboys recorded history for career total tackles. For his NFL career he was named to the 2013 class of Texas Gridiron Legends.
In February 2007, Dallas Cowboys head coach Wade Phillips announced the hiring of Nguyen as assistant linebackers coach and defensive quality control coach. Along with Phillips' son Wes, Nguyen was the first assistant hired under Phillips. Nguyen left the Cowboys on January 27, 2010.
Nguyen was named the inside linebackers coach at Texas A&M University on February 12, 2010. In Nguyen's first year as the A&M inside linebackers coach, the Aggies went 9-3 in the regular season with wins over nationally ranked Oklahoma and Nebraska as well as wins over Texas and Texas Tech. This marked the first time in school history that A&M had beaten all four of those teams in a single season. The resurgence of the Texas A&M defense to its former glory as "The Wrecking Crew" and particularly the outstanding play of the linebacker corps has been cited as a major reason for the team's 2010 season success.
Nguyen helped cement linebacker Von Miller's status in Texas A&M history by coaching him in the 2010 season which led to Miller being awarded the 2010 Butkus Award.
At the conclusion of the 2011 season, Dat Nguyen chose to leave Texas A&M, telling the Houston Chronicle's Brent Zwerneman that there was "no chance" of him being retained on Kevin Sumlin's staff.
Nguyen married his college sweetheart, Becky, and they have three daughters. In May 2004, Nguyen was awarded a Golden Torch Award at the Vietnamese American National Gala in Washington, D.C. He was also featured in HBO's Hard Knocks.