[13] 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 after Eric Berry and Joe Haden.

[14][15][16] On January 8, 2010, Thomas released a statement through the University of Texas which announced his decision to forgo his remaining eligibility and enter the 2010 NFL Draft.

"[19] He attended pre-draft visits and private workouts with multiple teams, including the Pittsburgh Steelers, Cleveland Browns, and Miami Dolphins.

[26] He made his professional regular season debut and first career start in the Seattle Seahawks' season-opener against the San Francisco 49ers and recorded seven combined tackles in their 31–6 victory.

Thomas made his first career interception off a pass by Chargers' quarterback Philip Rivers, that was originally intended for tight end Antonio Gates, and returned it for a 34-yard gain in the fourth quarter.

[29] In Week 12, Thomas collected eight combined tackles and returned a blocked punt for the first touchdown of his career during a 42–24 loss to the Kansas City Chiefs.

[30] Thomas started all 16 games during his rookie season in 2010 and recorded 76 combined tackles (64 solo), seven pass deflections, five interceptions, and a forced fumble.

[32] On January 9, 2011, Thomas started in his first career playoff game and recorded eight solo tackles and a pass deflection during a 41–36 victory against the New Orleans Saints in the NFC wild-card round.

Thomas intercepted a pass by quarterback Ryan Fitzpatrick, that was originally intended for tight end Scott Chandler, and returned it for a 57-yard touchdown in the third quarter.

[53] On October 28, 2013, Thomas collected a season-high ten solo tackles and made one pass deflection during a 14–9 victory at the St. Louis Rams in Week 9.

[60] On January 11, 2014, Thomas recorded 11 combined tackles (seven solo) and broke up two passes as the Seahawks defeated the New Orleans Saints 23–15 in the Divisional Round.

[62] On February 2, 2014, Thomas started in Super Bowl XLVIII and made seven combined tackles and a pass deflection during a 43–8 victory against the Denver Broncos.

[25][65] On November 9, 2014, Thomas recorded six combined tackles, deflected a pass, and made his only interception of the season during a 38–17 victory against the New York Giants in Week 10.

Thomas intercepted a pass by quarterback Eli Manning, that was intended for wide receiver Odell Beckham Jr., and returned it for a 47-yard gain in the fourth quarter.

[70] The Seahawks had the top-ranked defense in the NFL in fewest points allowed for the third straight season and finished atop the NFC West with a 12–4 record.

[71] On January 10, 2015, Thomas collected 11 combined tackles (five solo), two passes defended, and a forced fumble as the Seahawks defeated the Carolina Panthers 31–17 in the Divisional Round.

[72] The following week, he made five combined tackles, but suffered a dislocated shoulder in the second quarter of their 28–22 victory against the Green Bay Packers in the NFC Championship.

[84] On October 30, 2016, Thomas recorded two combined tackles, deflected a pass, and returned a fumble recovery for a touchdown during a 25–20 loss at the New Orleans Saints in Week 8.

Thomas recovered a fumble and returned it for a 34-yard touchdown after Cliff Avril stripped the ball from Saints' running back Mark Ingram II during the first quarter.

[87] On November 20, 2016, Thomas made four combined tackles and a pass deflection before exiting in the third quarter of the Seahawks' 26–15 win against the Philadelphia Eagles due to a hamstring injury.

[88] On December 4, 2016, Thomas suffered a broken tibia after he collided with teammate Kam Chancellor while breaking up a pass in the second quarter of the Seahawks' 40–7 victory against the Carolina Panthers in Week 13.

[92] He finished the 2016 season with 48 combined tackles (24 solo), a career-high ten pass deflections, two interceptions, a fumble recovery, and a touchdown in 11 games and 11 starts.

[94] Thomas started in the Seattle Seahawks' season-opener at the Green Bay Packers and collected a season-high 11 combined tackles (seven solo) and a pass deflection in their 17–9 loss.

[99] At the start of the 2018 season, Thomas did not report to training camp expressing that he would hold out until the Seahawks either renegotiated his current contract or traded him to another team.

[102] During a Week 4 matchup against the Arizona Cardinals, Thomas was carted off the field in the fourth quarter with a lower leg injury with an air cast attached to it, and gave "the finger" to the Seahawks' bench.

[107] After years of playing in the Seahawks' relatively simple Cover 3 scheme, Thomas initially found it hard to adjust to the Ravens' more complicated system.

[111] In Week 10 against the Cincinnati Bengals, Thomas recovered a fumble forced by teammate Chuck Clark on running back Giovani Bernard in the 49–13 win.

[112] In Week 14 against the Buffalo Bills, Thomas recorded a team high 7 tackles and sacked Josh Allen during the 24–17 win, clinching a playoff berth.

[114] In the divisional round of the playoffs against the Tennessee Titans, Thomas recorded a team-high seven tackles and sacked quarterback Ryan Tannehill during the 28–12 loss.

[118][107] His release came after players told coach John Harbaugh and general manager Eric DeCosta that they did not want Thomas back on the team.