Timmons was drafted by the Pittsburgh Steelers 15th overall in the 2007 NFL Draft. In his rookie season, he played in 16 games recording 13 tackles.
Timmons' playing time increased during his second season in 2008. He finished the season with 65 tackles, five sacks and an interception. In Super Bowl XLIII he recorded 5 tackles in the 27-23 victory over the Arizona Cardinals.
Timmons took over as starter for the 2009 season following the release of Larry Foote, who had wanted out due to Timmons's increasing playing time. Foote would return to the Steelers the following year to be Timmons' backup.
In 2010, despite getting less acclaim than many on the Steelers defense, he led the team in tackles with 135, also recording 3 sacks, 2 forced fumbles and 2 interceptions.
On August 23, 2011, the Steelers gave Timmons a six-year/$50 million contract extension. At the time, he was entering the last year of his rookie contract.