Helenius is a product of the Ilves Tampere organization in Finland. In 2002-2003 Helenius made his debut for Ilves' Junior C team where he played parts of two seasons. He split 2004-2005 between Ilves' Junior B and Junior A teams before making the full time jump to Junior A in 2005-2006 where he posted a 2.68 GAA with 4 shutouts and a .920 save percentage in 26 games. Helenius also dressed for 5 games at the SM-liiga level but did not play. In the summer, Helenius started for Finland in the U18 World Junior Championships where he caught the scouting world's attention going 4-1-1 with a 1.83 GAA, 1 shutout and .942 save percentage leading Finland to a silver medal. For his efforts, Helenius was named Best Goaltender in the tournament. At year's end Helenius was the 5th best goaltender by NHL Central Scouting and the 86th best prospect overall by Redline Report, but the Lightning selected Helenius in the 1st round, 15th overall of the 2006 NHL Entry Draft. In the Fall, Helenius appeared for the Lightning's Traverse City prospect team where he went 0-1-1 with a 3.84 GAA and .867 save percentage. Due to contractual and military service obligations, Helenius returned to Finland this season where he was expected to split time between the SM-liiga and Junior A. Unfortunately, after just 2 starts in Junior A where Helenius went 2-0-0 with a 2.00 GAA and .935 save percentage, Helenius suffered a season ending shoulder surgery which required surgery. In the summer of 2007, Helenius was selected by Seattle of the WHL in the 2007 CHL Import Draft with the 46th overall selection. The netminder appeared for the Lightning's 2007 Traverse City tournament team posting a disappointing 0-2-1 record with a 5.16 GAA and .818 save percentage. Helenius split starts most of the 2007-2008 junior season in Seattle with Jacob DeSerres, finishing the regular season with a 22-12-6 record with a 2.42 GAA and a .915 save percentage to go along with 3 shutouts in 41 appearances. In December he appeared for Finland at the 2007 U20 World Junior Championships posting a 2-2-0 record with a 3.36 GAA, a .863 save percentage, and 1 shutout as Finland finished a disappointing 6th in the tournament. In the WHL playoffs, Helenius redeemed himself helping the Thunderbirds to knock of Kelowna in the team's opening series. Helenius finished the playoffs with a 4-4-1 record with a 2.70 GAA and a .911 save percentage in 9 appearances.
Helenius was expected to play in Finland for two seasons while he fulfills his contractual and military service obligations in Finland. However, in the summer of 2007 Helenius signed a contract with the Lightning and committed to play for Seattle of the WHL. Expect Helenius to make his professional debut in the organization during the 2008-2009 season.