By 2002, his talent became evident when he won the ICA Junior Karting Championship in Finland, and successfully retained his title in 2003.

He broke the lap record at Snetterton, and achieved his first victory of the season in race two at the Thruxton round of the championship.

A win at Rockingham was the only major highlight, as he ended up sixth in the championship, losing out on fifth to Marcus Ericsson in the final round.

Mustonen moved to the Formula Three Euroseries for the 2009 season, driving for the Motopark Academy team alongside Christopher Zanella.

[1] Mustonen missed the rounds at Oschersleben due to health problems, and was replaced by Renger van der Zande.