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 four 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, 2005, 2007, 2008 and 2009 Mosconi Cup events.

More recently, 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. He has also won the China Invitational, the Pinneberg Open, the Salsburg Open, the Woensel Open and the Weert Kermis Tournament.

Niels Feijen has been under contract with cue manufacturer Longoni for several years.

In 2010, Feijen reached the finals of the WPA World Eight-ball Championship but ended up in 2nd place as he was dominated by Karl Boyes of Great Britain. He would reach the finals of that same tournament for the second consecutive time in 2011 but only to be defeated by Dennis Orcollo of the Philippines.