Happonen made his World Cup debut on 23 November 2001 in Kuopio, finishing an impressive eleventh in the individual large hill competition.

Five years later he scored his first World Cup win on 5 March 2006, in a team competition in Willingen.

This was followed up exactly a month later on 5 March, when Happonen won his first individual competition on the large hill in Lahti.

In 2006, Happonen won a silver medal in the team large hill competition at the 2006 Winter Olympics in Pragelato.

In the team large hill competition, Happonen produced an excellent jump of 139 metres (456 ft) in the second round, receiving judges' style points of 19 and 19.5, but this was not enough to earn the Finnish team a bronze medal.