Viljanen won the boys' singles event at the 1999 WDF World Cup in Durban, South Africa.
He was awarded a place in the 2015 PDC World Championship after Jarkko Komula, who had finished second on the Scandinavian Tour, was excluded following a suspension by the Finnish Darts Organisation.
[4] Viljanen won three SDC Tour events in 2015 and finished top of the Order of Merit, thus qualifying for the 2016 PDC World Championship.
[6] Viljanen made his debut at the 2016 World Cup of Darts, representing Finland with Marko Kantele and they lost 5–1 to Wales in the opening round.
[8] Viljanen missed two match darts in the first round of the World Cup as he and Kantele were knocked out 5–4 by Wales.