Born in Fulda, Hesse, Kehl joined Hannover 96's youth system in 1996 at the age of 15, going on to appear in two 2. Bundesliga seasons with the first team. In the 2000 summer he signed for SC Freiburg in the Bundesliga, making his debut in the competition on 12 August by playing the full 90 minutes in a 4–0 home win against VfB Stuttgart, and scoring his first goal on 12 December of that year to help to a 2–1 away success over VfL Wolfsburg.
Under legendary manager Volker Finke, Kehl featured mostly as a , helping the Black Forest club qualify to the UEFA Cup in his first season but suffering relegation in his second. He was no longer part of the squad, however, as he had left in the 2002 January transfer window to Borussia Dortmund, who went on to win the championship with 15 games and one goal from the player; previously, in summer 2001, he controversially agreed basic terms with FC Bayern Munich, and even accepted an advanced payment of 1.5 million Deutsche Mark, which was returned in November as Bayern threatened to take legal action.
During his tenure with Dortmund, Kehl was more often than not a starter. He contributed with 12 matches in the 2002–03 edition of the UEFA Champions League to help his team reach the second group phase, and amassed 61 league appearances from 2004 to 2006 in back-to-back seventh-place finishes.
In the first day of the 2006–07 campaign, Kehl injured his left knee in a challenge with Bayern's Hasan Salihamidžić, going on to be sidelined for several months and only recovering fully in the start of 2008–09, when he was made by new coach Jürgen Klopp. He eventually relapsed on his condition, also losing his importance in the starting XI after the emergence of Sven Bender and Nuri Şahin.
On 27 July 2013, Kehl played the second half of the DFL-Supercup contest against Bayern Munich, Bender in an eventual 4–2 win. In September, he was put out of action for another lengthy period after injuring his ankle in training.
Kehl began hinting at retirement in March 2014, shortly after having agreed to a one-year extension to his contract. The 35-year-old confirmed it at the end of the 2014–15 season, after having taken part in 21 games to help Borussia come from behind in the table and finally qualify to the Europa League.
In April 2018, Borussia Dortmund announced that Kehl would become its players licence department director starting on 1 June.