The big, hard fighting Dane was often ranked among the top few singles players of that era, but was rather overshadowed by his fellow countryman Peter Gade.

He has served the singles head coach of the Malaysia national badminton team since 4 January 2025.

[2] He played badminton at the 2004 Summer Olympics in men's singles, losing in the round of 32 to Chen Hong of China.

Jonassen reached the quarterfinals of several Badminton World Championships but never quite made it to the medal rounds.

He won the gold medal at the 2008 European Badminton Championships, late in his career.