Viljanen won the boys' singles event at the 1999
WDF World Cup in
Durban,
South Africa. In 2004, he progressed to the last 32 of the
World Masters, losing 3–2 to
Tony West. Viljanen competed on the Scandinavian Darts Corporation (SDC) Tour in 2014, and finished third in the Scandinavian Order of Merit. 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. He lost 4–1 in the preliminary round to
Sascha Stein. Viljanen won three SDC Tour events in 2015 and finished top of the Order of Merit, thus qualifying for the
2016 PDC World Championship. After winning 2–1 against Sven Groen in the preliminary round, he was beaten 3–0 by
Kevin Painter in the first round. 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. Another three SDC titles saw him qualify for the
2017 World Championship and he saw off Ross Snook 2–0 in the preliminary round, but could only win one leg against world number one
Michael van Gerwen during a 3–0 first round loss. Viljanen missed two match darts in the first round of the
World Cup as he and Kantele were knocked out 5–4 by Wales. ==World Championship results==