William spent the majority of his youth in the United States as the family followed Michael's career to Tampa Bay, Chicago, Washington, Boston, and New York. At age 14, the family relocated to Sweden — his father's native country — where William developed his game with clubs including Södertälje SK and Rögle BK before breaking into the Swedish Hockey League (SHL) as a 17-year-old with MoDo Hockey.
Nylander was selected by the Maple Leafs in the first round, eighth overall, of the 2014 NHL entry draft. Born in Canada, Nylander represents Sweden internationally. In the 2017 World Championship, he won gold and was named the tournament's most valuable player (MVP).