Pregraduate summer schools at the university and later on extended professional research stays abroad, helped him discover and build a diverse and lasting international network of colleagues.

Haagerup accidentally drowned on 5 July 2015, aged 65, while swimming in the Baltic Sea close to Fåborg where his family owned a cabin.

Following his appointment as professor at Odense, Haagerup got acquainted with Vaughan Jones, when he did research in Philadelphia and later at the UCLA in Los Angeles.

His solution to the so-called "Champagne Problem",[3] secured him the Samuel Friedmann Award in April 1985, although it was first published in Acta Mathematica in 1987.

Amongst the most academically prestigious were the Danish Ole Rømer Medal, the international Humboldt Research Award and the European Latsis Prize.