Nielsen was drafted by the New York Islanders in the third round, 87th overall in the 2002 NHL Entry Draft. After playing in the SwedishElitserien since 2001, Nielsen signed a two-year contract with the New York Islanders on 15 May 2006. He has also played for the Danish national team. With over 216 games in the Elitserien, Nielsen recorded 25 goals and 34 assists for 59 points and 66 penalty minutes. Nielsen started the 2006–07 AHL season with the Islanders' affiliate, the Bridgeport Sound Tigers, and was recalled to the Islanders on 5 January 2007. He made his NHL debut on 6 January. At the time of his recall, he had recorded a point in nine consecutive American Hockey League games, which was three games shy of the Sound Tigers' club record. During the 2010–11 NHL season Nielsen scored a league-leading seven short-handed goals, including two penalty shot goals in three-on-five shorthanded situations. Near the end of the 2010–11 NHL season Islanders' Captain Doug Weight was quoted as saying of Nielsen "He’s our best all-around player even though his stats don’t necessarily show it". On 7 February 2012, Nielsen signed a four-year, $11 million contract with the Islanders. Nielsen holds the distinction of being the most successful NHL player all-time in shootouts. He even scored a goal in a shootout prior to being credited with his first official NHL goal. His signature move in the competition has been dubbed the "Danish Backhand of Judgement", though he has also scored using his forehand. As of March 2013, he has a success rate of 62.5%. On 25 October 2014, he recorded his first career NHL hat trick against Anders Lindbäck of the Dallas Stars by scoring all three goals in the third period. in 2012 On 1 July 2016, Nielsen signed as a free agent to a 6-year contract with the Detroit Red Wings, marking the end of his 10-year tenure with the Islanders. During the 2016–17 season Nielsen was selected to participate in the 2017 NHL All-Star Game. During the 2017–18 season Nielsen scored his 47th shootout goal, making him the all-time leader in shootout goals. The goal came against the New York Rangers in a 3–2 win on 29 December 2017.
