In his rookie season, Jennings served as a nickelback and special teamer primarily until Marcus Trufant, Kelly Herndon, and Jimmy Williams all sustained injuries in week 16 and 17 of the 2006 NFL season. He ended up playing in 16 games, starting in 2, recording 40 tackles, 1 interception, 6 passes defended, 1 forced fumble, and giving up 2 touchdowns. In the postseason, with Seattle's secondary seriously hurting and forced to start safety Jordan Babineaux at #2 CB and previously out of the NFL Pete Hunter at nickel, Jennings was forced into the #1 role for the Wild Card game against the Dallas Cowboys and was primarily used to cover Terry Glenn due to their similar body build and playing styles. Glenn was held to 4 receptions for 41 yards, while Jennings recorded 1 tackle and a game-saving forced fumble on Glenn that would lead to a Seattle safety which would later prove vital in a 21-20 Seahawks victory. In the Divisional Playoff game against the Chicago Bears, Jennings recorded 6 tackles but was beat deep on a 68-yard touchdown pass from Rex Grossman to Bernard Berrian. The Bears ended up winning in overtime 27-24. Kelly Jennings had a great 2007 season as he finished with 55 tackles and 12 pass deflections. Coming into the 2008 season, Kelly had high expectations, but was moved to the nickelback and performed well once again at a position he has proven to be more than capable at. He finished the 2008 season with 42 tackles and 12 pass deflections.