In 2001, Feijen reached the finals of a nine-ball tournament in Tokyo, Japan. The event had a field of more than 700 players and offered the largest prize money at that time. However, he lost to Efren Reyes. In 2004, he won the inaugural Skins Billiards Championship with prize money of US$42,500. Feijen has won the European straight pool championship five times. In 2005 he was the winner of the Big Apple Nine-ball Classic, held in Queens, New York, an event with 128 of the world's best players. He represented Europe in the 2001, 2004-5, 2007-9 and 2011-16 Mosconi Cup events.
Feijen won the 2007 $50,000 winner-take-all International Challenge of Champions by defeating Lee Van Corteza. In 2008, Feijen won the World Straight Pool Championship with a victory over Francisco Bustamante in the finals. On October 5, 2008, he received the 3rd prize of $25,000 in the inaugural WPA World Ten-ball Championship.
In 2010, Feijen reached the finals of the WPA World Eight-ball Championship but ended up in 2nd place losing to Karl Boyes of Great Britain. He would reach the finals of that same tournament again the next year, and was defeated by Dennis Orcollo of the Philippines. 2014 has been the best year so far for Feijen, who won the European Straight Pool Championship, the European 9-ball Championship and the WPA World Nine-ball Championship. In 2015 he was voted Most Valuable Player of the Mosconi Cup for the fourth time in the last five years.
- WPA World Nine-ball Championship - Winner (2014)
- Mosconi Cup - Most Valuable Player (2011, 2013, 2014, 2015)
- World 8-ball Championship - Runner-up (2010, 2011)
- World 10-ball Championship - Third place (2008)
- World Straight Pool Championship - Winner (2008)
- European 9-ball Championship - Winner (2013, 2014)
- European Straight Pool Championship - WInner (2002, 2003, 2004, 2007, 2014)
- World Pool Masters - Winner (2013)
- insidepoolmag.com, Niels Feijen
- insidepoolmag.com, Finals at World 10-Ball Championship Set
- "REYES POCKETS POOL'S BIGGEST PAYOUT". Billiards Digest. Retrieved 2008-08-08.
- "By the Skins of His Teeth: Feijen Wins $42,500 at Skins Event Debut". AZ Billiards. Retrieved 2008-08-07.
- "Sports 123: Pool: European Championship: Men: 14.1". Sports123. Retrieved 2008-08-08.
- "Feijen Beats Corteza, Wins US$50,000 in International Challenge of Champions". AZ Billiards. Retrieved 2008-08-07.
- "Feijen is Predator World 14.1 Straight Pool Champion". Inside Pool Magazine. 2008-08-30. Retrieved 2008-08-31.
- billiardsdigest.com "Rule, Britannia! Appleton Wins World 10-Ball Championship"
- sports.inquirer.net, "Appleton Nips Wu for Title; Pulpul 4th"
- Jerry Forsyth (10 April 2010). "World 8-Ball Finals". AzBilliards.com. Retrieved 2010-04-11.
- WPA Staff (26 February 2011). "Orcullo Wins". AzBilliards.com. Retrieved 2011-02-26.
- Ted Lerner (27 June 2014). "The terminator snaps it off". World Poll Billiard Association. Retrieved 31 January 2016.
- "Europe have won the Mosconi Cup for the sixth time in a row". Sky Sports. 11 December 2015. Retrieved 31 January 2016.
-  WorldPoolMasters.com 2006 profile of Niels Feijen
-  AzBilliards.com August 14, 2005 report on Niels Feijen's win of the 2005 Big Apple Nine-ball Classic
- Niels Feijen 2008 World 14.1 Championship Winning Video
|WPA World Straight Pool Champion
|WPA Men's World Nine-ball Champion