Steen began his professional hockey career by playing for Frölunda HC between the years of 1999 and 2004. He spent the 2004-05 season season with MoDo Hockey after a highly controversial signing.
In the 2002 NHL Entry Draft, Steen was selected in the first round, 24th overall, by the Toronto Maple Leafs. He played his first game with the Maple Leafs on October 5, 2005 during the season opener against the rival Ottawa Senators, and registered a minor penalty. His first career NHL goal came in the next game on October 8 against the Montreal Canadiens. This goal marked the first time a Swedish father and son both scored in the NHL.
Steen scored his first career hat-trick on January 4, 2007 against the Boston Bruins, ending the night with a 5 point game.