On June 23, 2007, Grimes appeared in World Bowl XV and was part of the Hamburg Sea Devils team that defeated the Frankfurt Galaxy 37–28.

[5] After winning World Bowl XV, Grimes returned to the Atlanta Falcons and attended organized team activities and training camp.

[7] On December 17, 2007, the Atlanta Falcons promoted Grimes to their active roster after cornerback Lewis Sanders tore his pectoral muscle and was placed on injured reserve.

Petrino decided to accept the vacant head coaching job at Arkansas after quarterback Michael Vick was arrested for operating an unlawful interstate dog fighting venture.

[9] Interim head coach Emmitt Thomas named Grimes the third cornerback on the depth chart, behind DeAngelo Hall and Chris Houston.

[10] On December 23, 2007, Grimes made his professional regular season debut and recorded a season-high 11 combined tackles (ten solo) during a 30–27 loss at the Arizona Cardinals in Week 16.

The Atlanta Falcons' new head coach, Mike Smith, named Grimes a starting cornerback to begin the 2008 regular season, along with Chris Houston.

[15] During training camp, Grimes competed for a job as a starting cornerback against Chris Owens, Von Hutchins, Chevis Jackson, and David Irons.

[20] Head coach Mike Smith named Grimes and Chris Houston the starting cornerbacks to begin the regular season.

[21] On November 22, 2009, Grimes collected a season-high ten combined tackles (eight solo), two pass deflections, and an interception during a 34–31 loss at the New York Giants.

[26] Head coach Mike Smith named Grimes and newly acquired free agent Dunta Robinson the starting cornerbacks to begin the 2010 regular season.

[27] On October 17, 2010, Grimes collected a season-high 12 combined tackles (ten solo) and a pass deflection in the Falcons' 31–17 loss at the Philadelphia Eagles in Week 6.

On January 15, 2011, Grimes started in his first career playoff game and collected seven solo tackles in a 48–21 loss to the Green Bay Packers in the NFC Divisional Round.

[34] Defensive coordinator Brian VanGorder retained Grimes and Dunta Robinson as the starting cornerback duo to begin the regular season.

[36][37] Grimes re-injured his knee and was inactive for the Falcons' Week 17 victory against the Tampa Bay Buccaneers and during their NFC Wildcard Game at the New York Giants.

[40][41] On April 24, 2012, Grimes signed his one-year, $10.60 million franchise tag after speculation that the Falcons were seeking a trade to acquire cornerback Asante Samuel.

[34][42] The Atlanta Falcons' new defensive coordinator Mike Nolan retained Grimes and Dunta Robinson the starting cornerbacks to begin the season, ahead of Asante Samuel.

[43] He started in the Atlanta Falcons' season-opener at the Kansas City Chiefs and recorded six solo tackles before exiting in the fourth quarter of their 40–24 victory with a calf injury.

[44] On September 11, 2012, the Atlanta Falcons placed Grimes on injured reserve for the rest of the season after it was discovered he would have to undergo surgery for a torn Achilles tendon.

[52] In Week 9, he collected five solo tackles, deflected a pass, and returned an interception for his first career touchdown in the Dolphins' 22–20 win against the Cincinnati Bengals.

[63] On October 26, 2014, Grimes made three combined tackles, a pass deflection, and returned an interception for a touchdown during a 27–13 win at the Jacksonville Jaguars in Week 8.

[67][68] He made an impressive one-handed interception off a pass attempt by Matthew Stafford that was originally intended for wide receiver Calvin Johnson in the second quarter.

[79] After signing with the Tampa Bay Buccaneers, his wife made a post on Instagram where she described the Miami Dolphins as "the dumpster fire of the AFC East.

Head coach Dirk Koetter named Grimes and Alterraun Verner the starting cornerbacks to begin the regular season, ahead of Vernon Hargreaves, Josh Robinson, Jude Adjei-Barimah, and Johnthan Banks.

Grimes completed the 2016 season with 57 combined tackles (51 solo), a career-high 24 pass deflections, four interceptions, a forced fumble, and a touchdown in 16 games and 16 starts.

On October 15, 2017, Grimes recorded a season-high eight combined tackles, deflected two passes, and returned an interception by quarterback Carson Palmer for a 39-yard gain during a 38–33 loss at the Arizona Cardinals in Week 6.