Timmons attended Wilson High School in Florence, South Carolina, where he played football and ran track.

During his senior year, Timmons registered over 150 tackles and two sacks, while also catching 47 passes for over 800 yards and five touchdowns as a tight end, which earned him all-state first-team honors.

He was awarded the Defensive Player of the Year and received Super Prep Elite 50 honors, as well as selected to play in the 2003 East-West Shrine Game.

In his true freshman year, Timmons played in all 12 games including the Seminoles' Gator Bowl victory over West Virginia, although only limited minutes mainly on special teams and as a backup linebacker.

As a sophomore, Timmons ranked second on the Seminoles' depth chart at strongside linebacker behind Ernie Sims.

Timmons finished the year with 35 tackles (17 solo), which ranked third on the team (behind Buster Davis and Sam McGrew), as well as three quarterback sacks, which also was third-best among all Seminoles (behind Kamerion Wimbley and Marcello Church).

His season-best performance came in the ACC Championship Game against Virginia Tech, when he recorded eight tackles, six solo, two quarterback sacks and two QB hurries.

He had a productive season, recording 79 tackles and five sacks, and was honored with All-American third-team and All-Atlantic Coast Conference first-team.

On November 3, Timmons earned his first career start in a victory over the Washington Redskins during Monday Night Football and finished the game with four combined tackles.

On January 11, 2009, he played in his first career playoff game when the Steelers defeated the San Diego Chargers 35–24 in the wild card round.

The Steelers defeated the Baltimore Ravens in the AFC Championship and went on to play the Arizona Cardinals in Super Bowl XLIII.

He finished the 2009 season with a total of 78 combined tackles, 7.0 sacks, 4 pass deflections, and 3 forced fumbles in 14 games with 13 starts.

In 2010, he had a career year for the Steelers and finished off the season as the highest-rated inside linebacker by Pro Football Focus.

[15] His rookie deal expired at the end of the 2010 season and he was a free agent; however, the Steelers signed him to an extension after the NFL lockout was resolved.

On November 25, 2012, Timmons racked up ten combined tackles and intercepted Browns' quarterback Brandon Weeden, and scored a 52-yard touchdown for the first of his career.

On January 17, he made six combined tackles in a 23–16 divisional loss to the eventual Super Bowl champions, the Denver Broncos.

[9] He finished the season with 114 combined tackles, 2.5 sacks, 5 pass deflections, two interceptions, and a forced fumble in 16 games with 16 starts.

[26] On January 8, 2017, Timmons recorded a team-leading 14 combined tackles and sacked quarterback Matt Moore twice in a 30–12 AFC Wild Card Round victory over the Miami Dolphins.

[27] On January 22, Timmons repeated a team-leading performance of 14 combined tackles in a 36-17 AFC Championship loss to the eventual Super Bowl champions, the New England Patriots.

[29][30] On September 19, the Dolphins suspended Timmons indefinitely after he went missing before the team's Week 2 match-up against the Los Angeles Chargers.