[2][3] Kahlenberg progressed through the ranks at the club's youth system and said that he learned a lot from development to "behaving as a professional footballer".

[7] At the start of the 2002–03 season, Kahlenberg played in the central midfield for Brøndby, where he set up the attacks with his great technique and view for the game.

[8] Kahlenberg made his UEFA Champions League debut, coming against Dinamo Tirana in the first leg of the tournament second round, starting a match and played 67 minutes before being substituted, in a 1–0 win.

[15] His performance attracted interests from European clubs, leading his agent stating Kahlenberg is worth 5 million kroner.

[23] Amid to serving a one match suspension, Kahlenberg played in both legs of the UEFA Cup Qualification Round, as he helped the club win 5–0 on aggregate against Dinamo Minsk.

[30] Kahlenberg also played an important role in the UEFA Cup matches when he helped the club beat Viktoria Žižkov and Schalke 04.

[38] However, Kahlenberg was sent–off for a second bookable offence against FK Ventspils in the second leg of the UEFA Cup second round qualification, as the club were eliminated from the tournament.

[41] Cutting into the field from his wide position, Kahlenberg scored eight more goals, including a two braces against København and Herfølge Boldklub.

[67] On 6 January 2007 he scored on his return to the starting line–up against Chamois Niortais in the first round of Coupe de France, as the club lost 5–4 in a penalty shootout following a 2–2 draw.

[89] In May 2009, Kahlenberg signed a four-year contract with the defending German champions Wolfsburg for a €4 million transfer fee, effective from the end of the season.

[101] In mid-February, both he and Alexander Madlung, were suspended by Wolfsburg's interim manager Pierre Littbarski, citing lack of commitment.

[102] A month after his suspension, Kahlenberg went on trial with English Premier League side Blackburn Rovers, but did not earn a contract.

Ahead of the 2011–12 season, Kahlenberg was considering leaving Wolfsburg as a result of lack of first team opportunities, but ended up staying at the club.

[110] Kahlenberg was given a number 38 shirt and made his first appearance of Wolfsburg, starting the whole game, in a 0–0 draw against FC Augsburg on 14 September 2012.

[117] In January transfer window in 2012, it announced that Kahlenberg has returned to Ligue 1, but joined Evian on loan, until the end of the 2011–12 season.

[118] Upon moving to the club, he was joined by compatriots Daniel Wass, Stephan Andersen and Christian Poulsen and was also given the eleven number shirt.

However, his hopes of permanent move to the club was apparently broken down after Wolfsburg's reluctance to discuss a fee, resulting in its cancellation.

[131] Kahlenberg made his second debut for the club, starting a match and playing 88 minutes before being substituted, in a 0–0 draw against Randers on 25 August 2013.

[141] After missing a match due to suspension, Kahlenberg returned to the starting line–up against Nordsjælland in the last game of the season and set up two goals, as Brøndby drew 2–2.

[149] Reflecting in the first half of the season, Kahlenberg said: "We have a high top level, and on our good days we are perhaps the best playing team in Denmark.

[155] Kahlenberg captained his first match of the season, coming against FC Midtjylland on 25 October 2015 and set up Brøndby's second goal of the game, in a 2–1 win.

[164] After returning to play for the club's reserve, Kahlenberg made his first league appearance of the season, coming on as a second-half substitute, in a 3–0 loss against SønderjyskE on 21 May 2017.

[170][171] In September 2002, Kahlenberg, who was recognized as one of the most talented Danish midfielders, was called up to Denmark under-21 national team for the first time.

[173] Later in the year, Kahlenberg played in both legs of the UEFA European Under-21 Championship qualification play-offs against Italy U21, as the U21 side lost through away goal following a 1–1 draw.

[170] Even though Denmark was eliminated in the preliminary group stage, Kahlenberg managed to score three goals in three games, and he was selected for the "Team of the Tournament".

[170] In April 2003, the senior national team coach Morten Olsen called Kahlenberg to the squad for the first time.

[191] In May 2010, Morten Olsen announced that Kahlenberg would be part of the final squad of 23 participating in the FIFA World Cup in South Africa.

[192] He made his tournament debut against Netherlands, starting a match and played 73 minutes before being substituted, as Denmark lost 2–0 on 14 June 2010.

[216] On 5 March 2020, Kahlenberg tested positive for the novel coronavirus disease called COVID-19 during the current pandemic, after having been infected at a birthday party in Amsterdam.

[218] Peter Madsen, another retired Danish football player who attended the same Amsterdam birthday party as Kahlenberg, also tested positive for the coronavirus.