Bille Nielsen started his senior career with
Danish 1st Division club
BK Frem. He was promoted to the first-team in the summer 2005, and made his debut on 31 July 2005 against
Fremad Amager. He signed a two-year contract with Frem in October 2005, and was on trial with English club
Newcastle United in November 2005, Though he never secured himself in the Frem starting line-up, under manager
Walter Mazzarri. He scored in his debut game for Reggina, a
Coppa Italia game on 8 November. He got his Serie A debut on 19 November, and played seven games during the
2006–07 Serie A season without scoring any goals. He also played for Reggina's youth team, helping them advance in the 2007
Campionato Nazionale Primavera by scoring two goals in two games against
A.C. Milan Primavera. Under new manager
Massimo Ficcadenti, Bille had a difficult time securing a place in the starting eleven, and he underwent loan deals with
Serie C1 teams
A.C. Martina and
A.S. Lucchese-Libertas. Both clubs went bankrupt during his stays, and according to his agent Bille did not get his wage from Reggina either. In August 2008, Bille agreed to rescind all claims of the wages owed to him, in order for his contract to be terminated a year before its expiration. He moved back to Denmark, and trained with
Danish Superliga team
FC Nordsjælland in August 2008. On 18 September the
Danish Football Association ruled that in order to help him re-establish his career, Bille's eight-day suspension would expire from the ninth match day of the
2008–09 Danish Superliga season even though he had not been signed by any club, making him available for selection from the start of October. He signed a contract with Nordsjælland on 25 September, and made his debut in October. He played 10 games in his first season with Nordsjælland, as he was backup for
Martin Bernburg. As Bernburg was sold, Bille got his breakthrough in the
2009–10 Danish Superliga season, Nicki Bille gained his first team debut 5 February 2011 in a home fixture against Levante FC, Villarreal CF lost the game 0–1. He was loaned out to
Rayo Vallecano for the 2012–2013 La Liga season. On 3 January 2013 he signed a four-year deal with Norwegian side
Rosenborg after ending his stay at
Rayo Vallecano earlier than planned. Billie Nielsen changed from number 9 to his favorite number 14 before the 2014 season. In 2014, Rosenborg sold Nielsen for an undisclosed amount to the French club
Evian Thonon Gaillard F.C. During his Rosenborg career, he had scored 24 goals in 50 games. On 1 July 2015, he signed a three-year contract with
Esbjerg fB, but his time at Esbjerg was never a success. He left half a year after signing with the club having played 19 games and scored five goals. On 27 January 2016, he signed a 2.5-year contract with an option of one additional year with Polish champion
Lech Poznań. He was given shirt number 19 after the former topscorer
Kasper Hämäläinen. Bille got an injury in February 2016 in a match against
Podbeskidzie Bielsko-Biała, where he only played 25 minutes before leaving the pitch. He was out for 1 month . In August 2016, Bille suffered an injury in his
groin that kept him out for the next seven months. On 30 January 2018,
Panionios F.C. officially announced the purchase of experienced Danish international striker Nicki Niels Bille Nielsen from
Lech Poznań, for an undisclosed fee. In July 2018 he signed a contract with
Lyngby Boldklub, who was managed by Bille's former youth coach
Mark Strudal. On 26 October 2018 he was sacked from Lyngby following an episode, where he was arrested and charged by the
Copenhagen Police Department. On 25 July 2019, Bille joined
Ishøj IF in the
Denmark Series – the 4th tier of Danish football. He was released without making an appearance, on 20 November 2019. In June 2020 he joined
VB 1968 in the
Zealand Series, the fifth tier of Danish football. At VB 1968 he joined several other former professional footballers such as
Martin Albrechtsen,
Quincy Antipas and
Nabil Aslam. ==International career==