A central defender, he was capped 51 times for the Danish national team, and represented his country at 1986 World Cup and two European Championship tournaments.

Born on Frederiksberg in Copenhagen, Ivan Nielsen started his senior career with Fremad Amager in 1975.

In 1979, he moved abroad to play professionally with Feyenoord Rotterdam in the Dutch Eredivisie championship.

Moving on to defending Eredivisie champions PSV Eindhoven, Nielsen joined up with fellow Danish internationals Frank Arnesen and Jan Heintze.

In a 1986 World Cup match, he successfully marked Uruguayan star Enzo Francescoli, and left him unable to move and play his football.

The referee was close to the situation and awarded a penalty, on which Francescoli scored Uruguay's only goal (2–1) in Denmark's 6–1 win.