Also a standout athlete, Thomas was on the school's track & field team, where he competed as a sprinter and jumper, and was a member of the 4 × 200 meters relay team that reached the state finals, at 1:27.92. He finished second in the long jump at the 2007 Region 3-3A Meet, with a personal-best mark of 7.14 meters.
Considered a four-star recruit by Rivals.com, Thomas was ranked as the No. 12 athlete in 2007.
Thomas attended the University of Texas at Austin, where he played for coach Mack Brown's Texas Longhorns football team from 2007 to 2009. After redshirting his first year at Texas, Thomas started all 13 games at strong safety for the Longhorns in 2008, and ranked second on the team with 63 tackles and 17 pass breakups, the most ever by a Longhorn freshman. He also had two interceptions, four forced fumbles, and a blocked kick.
Thomas subsequently earned multiple All-Freshman honors, as he was named to FWAA′s Freshman All-America team,Sporting News′ Freshman All-American team,College Football News′ All-Freshman first team, and Rivals.com′s Freshman All-America team.
As a redshirt sophomore in 2009, Thomas intercepted 8 passes, returning two of them for touchdowns. The Longhorns were undefeated in the regular season and Thomas played in the 2010 BCS National Championship Game where they lost to Alabama.
Thomas chose to forgo his final two seasons of eligibility at Texas to declare for the 2010 NFL Draft where he was the third defensive back taken.
Thomas was chosen by the Seattle Seahawks with the fourteenth overall pick in the 2010 NFL Draft. At age 20, he was one of the youngest players eligible for the draft. Mike Mayock, longtime draft analyst for NFL Network, refers Earl Thomas as having the best chance of resembling Ed Reed in the 2010 NFL Draft class.
On July 31, 2010, Earl Thomas agreed to a five-year, $21.1 million contract with the Seahawks, receiving $12.31 million in guarantees.
On September 26, 2010 in a Week 3 game against the San Diego Chargers, Thomas intercepted two passes by Chargers quarterback Phillip Rivers, one of which sealed a 27–20 victory for the Seattle Seahawks. Thomas finished the season with 76 tackles, one forced fumble and five interceptions that helped the Seahawks into the playoffs.
Thomas in the 2011 preseason.
In the 2011 regular season, Thomas was a key player on the Seahawks defense. The Seahawks finished the year with the second-fewest 20-yard pass plays surrendered, largely thanks to Thomas playing single high safety, where he defends the deep portion of the field. He finished the season with 98 tackles and 2 inteceptions. His play earned him a Pro Bowl nomination and second-team All-Pro selection. He was also ranked No. 66 in the 2012 NFL Top 100.
In the 2012 season, during Week 3 in the pre-season in an away game against the Kansas City Chiefs in Kansas City, Thomas intercepted a Matt Cassel pass and returned it 75 yards for a touchdown as the Seahawks won the game 44-14.
Thomas finished the season with 66 tackles and 3 interceptions. On January 2, He was selected to the 2013 All-Pro Team.
In the 2013 season, the Seahawks won 13 games that tied the team record. Thomas finished the season with 105 tackles and 5 interceptions. The Seahawks reached Super Bowl XLVIII where they would face the Denver Broncos and the record breaking offense. Seattle defeated Denver and held the record breaking offense to 8 points. The score was 43-8 and Thomas had 7 tackles.
On April 28, 2014 Thomas agreed to a four-year contract extension worth $40 million with $27.725 guaranteed, making him the highest paid safety in the NFL.
On September 1, 2014 head coach Pete Carroll announced that Thomas would be the primary punt returner for the Seahawks entering the 2014 season. Thomas was then removed from the punt returner position in favor of Bryan Walters. Thomas responded saying they removed him from the position because of a risk of injury, and allow him to focus "on defense".
Thomas would again make the All-Pro team and the Pro Bowl in 2014, recording 97 tackles, 2 tackles for loss, forcing 3 fumbles, 1 interception and 6 passes defended in 16 games. The Seahawks had the #1 defense in the NFL in fewest points allowed for the third straight season, Thomas starting in every game over those three seasons. Thomas would have 11 tackles, 2 passes defended, and a forced fumble in the NFC divisional round against the Carolina Panthers, then record 5 tackles in the NFC Championship game against the Green Bay Packers. Thomas suffered a dislocated shoulder in the second quarter of the game, but would return to the game in the second half. He helped the Seahawks win 28-22 to advance to the Super Bowl for the second straight season. In Super Bowl XLIX, Thomas recorded 9 tackles in a 28 – 24 loss to the New England Patriots.