[1] He held the IBO heavyweight title from 1996 to 1999 successfully defending it five times, including against Larry Holmes and Phil Jackson, the second highest number of defenses behind Wladimir Klitschko's record of 18.

As an amateur, Nielsen won bronze medals in the super heavyweight division at the 1991 European Championships and the 1992 Olympics in Barcelona.

After his win over 41-year-old ex-World Boxing Association champion James 'Bonecrusher' Smith in October 1994, Nielsen began to attract attention.

[2] On February 8, 2010, Brian Nielsen told reporters that he was prepared to return to the ring, after 8 years of absence,[3] and that a proposed fight against Evander Holyfield had been met with interest by both sides.

Controversy arose in early 2004, when journeyman heavyweight Thomas Williams stated that he had been bribed to throw his fight against Nielsen in March, 2000.

Nielsen and Mogens Palle denied any knowledge of the match fixing, and were not charged of any wrongdoing by American or Danish authorities.