Born in Miami, Florida, Thomas was raised by his mother. His father, Air Force Captain and B-52 pilot Robert James Thomas, died during a mission in the Vietnam War. Thomas started playing football when he was three years old. He played high school football at South Miami Senior High School.

Alongside Cornelius Bennett and later Keith McCants, Thomas spearheaded one of the best defensive lines in college football and smashed many Crimson Tide defensive records, including sacks in a single season. He was awarded the Butkus Award in 1988 after a season which saw him set an NCAA record 27 sacks along with finishing 10th in Heisman Trophy balloting. He currently holds the single season NCAA FBS sack record with 27 and what was the career sack record with 52 career sacks. He was also selected as a unanimous All-American at the conclusion of the 1988 season, a season which culminated in the Crimson Tide’s thrilling 29-28 victory over Army in the 1988 Sun Bowl. In 2000, Thomas was named a Sun Bowl Legend. He was awarded the Sington Soaring Spirit Award by the Lakeshore Foundation. This annual award is named for University of Alabama football legend Fred Sington. Thomas was elected to the College Football Hall of Fame in 2014.

Thomas was selected in the first round of the 1989 NFL Draft, fourth overall, and was signed by the Chiefs. He would remain with the Chiefs for his entire career.

Thomas’s rookie year was very successful, earning him Defensive Rookie of the Year by the Sporting News, and was the first Chiefs’ linebacker to be elected to the Pro Bowl in his first season since Hall of Fame player Bobby Bell. He would appear in nine Pro Bowls during his career.

Thomas was perhaps most well known for his ability to sack the quarterback and was named an All-Pro 9 times and was voted to 9 Pro Bowls in his 10 year career. He totaled 126.5 sacks in his career, ranking 12th all-time in NFL history, and still holds the single game record of 7 quarterback sacks, a feat which occurred against Seattle’s Dave Krieg on Veterans Day, 1990. Ironically, it was a sack that Thomas didn’t get that decided the game: on the final play, Krieg eluded a blitzing Thomas and threw a touchdown pass to give the Seahawks a 17-16 win. The next player to come close to breaking this record was Thomas himself, recording 6 sacks against the Oakland Raiders in the regular season opener in 1998.

He is one of only 25 NFL players to achieve 100 or more sacks, and ranks fifth all-time in Chiefs’ history with 649 career tackles. During his career, he also recorded 1 interception and recovered 19 fumbles, returning them for 161 yards and 4 touchdowns. Thomas established Chiefs career records for sacks, safeties, fumble recoveries, and forced fumbles. Off the field, Thomas established the Third and Long Foundation in 1990, which helps inner city youth learn to read and teaches life skills through educational and cultural programs.

On January 31, 2009, Thomas was elected to the Pro Football Hall of Fame in his fifth year of eligibility.

On January 23, 2000, Thomas’ 1999 Chevrolet Suburban went off Interstate 435 as he and two passengers were driving to Kansas City International Airport during a snowstorm, where he was going to fly to St. Louis to watch the NFC Championship game. Police reports indicate that Thomas, who was driving, was speeding at approximately 100 m.p.h. even though snow and ice were accumulating on the roadway. Thomas continued weaving erratically through traffic until the time of the accident. Thomas and one of the passengers were not wearing seat belts and both were thrown from the car; the passenger was killed instantly. The second passenger, who was wearing his safety belt, walked away from the scene uninjured. Thomas was left paralyzed from the chest down. By early February, Thomas was being treated at Miami’s Jackson Memorial Hospital. The morning of February 8, 2000, while being transferred from his hospital bed to a wheelchair on his way to therapy, Thomas told his mother he was not feeling well. His eyes then rolled back, recalled Frank Eismont, an orthopedic surgeon at Jackson Memorial Hospital. Eismont said Thomas went into cardio-respiratory arrest and died as a result of a pulmonary embolism; a massive blood clot that developed in his paralyzed lower extremities and traveled through his venous system to his lungs. Months later, Thomas’ mother, Edith Morgan, his seven children and their five mothers sued General Motors in an attempt to make them take financial responsibility for the accident that Thomas caused, seeking at least $73 million. In 2004, a jury ruled the family was not entitled to any money.