[3] After graduating from Frederiksberg Gymnasium in 1931, he started studying physics under Niels Bohr and chemistry at the Technical University of Denmark.

He started his career with working at AarhusKarlshamn 1936–1939 before founding his own catalysis company Haldor Topsøe in 1940 with which he established himself as engineer in Kongens Lyngby.

During the 1950s, Topsøe experimented with petition of silicon to semiconductors in the cellar in his home and in 1959, he founded the process Topsil.

Topsøe was also member of several organizations, among them the Royal Swedish Academy of Engineering Sciences and since 1951 Akademiet for de Tekniske Videnskaber.

In early May 2013, Topsøe broke his hip following a fall and had to cancel his planned centenarian celebration.