He began playing football at [Rosenhoj BK] in Denmark, after that he played for Danish clubs Hvidovre IF and Brøndby IF. The midfielder made his debut for Ajax on 17 September 2006 in the league match Roda JC - Ajax (2-0). Krohn-Dehli played for a number of Danish clubs before joining Ajax from Brøndby IF. Krohn-Dehli excelled as a youth team player for Ajax under Danny Blind. In 2004 however, he was allowed to leave Ajax by manager Ronald Koeman and he joined RKC Waalwijk on a free transfer. There he made his professional debut on August 15, 2004 in a goalless draw against FC Groningen. Two years later, he was re-signed by Ajax on a free transfer until 2009. He made his debut for the Denmark national football team in an Euro 2008 qualification match against Liechtenstein on October 11, 2006. After the winter break with only three appearances for Ajax he was sent on loan to Sparta Rotterdam on January 31, 2007 for the remainder of the season. Five days later he made his first out of 12 appeareances for Sparta against Roda JC. After his spell at Sparta and the summer break he returned to Ajax for the 2007-08 season.
Ajax announced on August 29 2008 that they would transfer Krohn-Dehli to Danish Brøndby IF.. He made his debut two days later on the 31. august against FC Copenhagen. Brondby won 1-0.
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