Poulsen's route to Anfield was certainly a circuitous one. His aggressive playing style has seen him involved in a number of controversial incidents over the years. Poulsen was given his first big break as a twenty-year-old by Roy Hodgson at FC København at the start of the new millennium; and was part of the squad that won the Danish Superliga title in 2001 and ended as runners-up in 2002 when he was voted the club's Player of the Season. Poulsen played as an attacking midfielder in Denmark, scoring ten goals in 45 games for the team from Copenhagen, before moving to German club Schalke after the 2002 World Cup. He stayed in Gelsenkirchen for four seasons, adding the German league cup to his list of honours, before moving in 2006 to the reigning UEFA Cup holders, Sevilla. Poulsen was the first player to be named as the Danish Player of the Year for two consecutive years in 2005 and 2006 and arrived with a big reputation to protect. Poulsen was a phenomenal success at Sevilla and Spanish newspaper Marca singled him out as the best buy in Spain over the summer. The Spanish club retained the UEFA Cup in Glasgow after defeating their compatriots Espanyol; the match went to penalties and Poulsen played throughout the 120 minutes.
After two seasons in Spain the Dane moved again, this time to Juventus. Although he signed a four-year contract on his arrival in Italy, he would only spend two seasons with the Turin club featuring in 48 out of their 76 league games. Juve were prepared to release him after a single season with Turkish clubs interested, but he stayed on to fight for his place. Poulsen is certainly a take-no-prisoners type of player, uncompromising being a common adjective for his style of play. He had famous battles with Totti and Kaká with the former losing his temper and spitting in Poulsen's face in the 2004 European Championship earning himself a three-match ban in the process. Then Milan coach, Carlo Ancelotti, branded Poulsen a coward after Schalke's and Milan's duel in the 2005/06 Champions League, claiming: "He plays his game when the referee is not watching, he starts swinging kicks, pushing his rivals when the official's back is turned. He is a coward and he shouldn't be playing football." Arriving on Merseyside was seen by many of the supporters as one of the main reasons why Roy Hodgson's team laboured so much. Poulsen's compatriot, Daniel Agger, warned Poulsen that English football was quite different to what he had experienced. "Maybe he will find the pace of here a bit of a culture shock; he will have to get used to the extra-high tempo and he won’t be able to drink café lattes like he does in Italy." Poulsen clearly struggled to cope with the English game, to say the least! Being raised in later years on Graeme Souness, Steve McMahon and Javier Mascherano patrolling the midfield, Poulsen's limp play was a sorrowful sight for any Liverpool fan. Poulsen signed a one-year deal with French Ligue 2 champions Evian Thonon Gaillard in the summer of 2011. Evian acquitted themselves well in the top division, eventually finishing in ninth position, and Poulsen appeared in 24 of the 38 league fixtures. In doing so, he became only the second footballer, after Romanian Florin Raducioiu, to have played professionally in the top division of all five of the so-called big European Leagues: Germany, Spain, Italy, England and France. Poulsen added the Netherlands to his curriculum vitae when he joined Ajax in August 2012.
Poulsen appeared in thirty-seven first-team matches for Ajax in 2012/13. The Amsterdam club won the Dutch championship for the thirty-second time. It was the first championship-winning club that the Dane had been actively involved with for twelve years.
The Danish midfielder continued to be a regular member of the Ajax team in 2013/14, appearing in 29 of the 34 league matches as the Amsterdam club retained its domestic title. Poulsen also appeared in all 8 of his club's European matches during the season. He then returned to Denmark with FC Copenhagen on a one year deal, helping them to the Danish Cup. He failed to find a club in the next three transfer windows and in December 2016 his agent formally announced he had retired from playing.
Poulsen coached at Danish side B93 for two years and then took a role at previous club Ajax. In 2019 he became Erik Ten Hag's assistant, but didn't follow him when he left for Manchester United in 2021.