After playing two seasons for RKC he moved to Hamburg to play in the German Bundesliga at the start of the 2004–05 season.
While at Hamburg, he earned his nickname "Khalid der Kannibale" ("Khalid the Cannibal") for his ability to eat up the opposition.
Boulahrouz was born in Maassluis, Netherlands to a family of Moroccan descent. As a youngster he went to the youth academies of Ajax and Haarlem.
When he was sixteen his father died and he had to take responsibility for his family. While Boulahrouz was in Switzerland preparing for the Euro 2008 quarter finals against Russia, his then wife Sabia gave birth to a prematurely born daughter Anissa who died in a Lausanne hospital. In January 2013 the couple announced its separation and Sabia has since formed a relationship with fellow player Rafael van der Vaart.
He played top-flight football in the Netherlands, Germany, England, Spain, Portugal and Denmark.
A full international from 2004 to 2012, he earned 35 caps and was included in the Dutch squads for two World Cups and two European Championships.
He wore the number 9 shirt, previously occupied by Hernán Crespo – this is an unconventional number for a defender, being traditionally associated with strikers, but was given to him just because it happened to be one of the numbers not currently allocated to a player by Chelsea at the time of signing.
After splitting from Sabia, he began dating Miss Nederland 2014 Yasmin Verheijen in October 2014.He began his professional career in the Dutch Eredivisie for RKC on 9 March 2002, against Heerenveen.This is illustrated by the amount of cards he received – 16 yellows and three reds in two seasons at the club.He was also part of a defence that conceded the least amount of goals in the 2005–06 Bundesliga season: 30 goals in 34 games.So, for example, now I can have [Khalid] Boulahrouz, Ricardo [Carvalho] and John [Terry] — three options for the centre; I can have Paulo [Ferreira] and Boulahrouz – two options for the right side; I can have Boulahrouz and Wayne Bridge — two options for the left side." On 21 August 2006, Chelsea completed the signing of Boulahrouz.Six days later, he was handed his Premiership début against Blackburn Rovers.