2010-11: William Nylander returned to America from Sweden and played for the Chicago Mission U14 bantam team that reached the USA Hockey championship game. In 29 regular season games he scored 34 goals with 27 assists and had 8 penalty minutes. Nylander was the second-leading scorer for Chicago Mission at the Tier 1 National Championship behind Seamus Malone — scoring 5 goals with 4 assists in six games. Chicago Mission lost to Belle Tire of Detroit, 6-5, in the championship game.
2011-12: Nylander moved back to Sweden with his family after his father, Michael Nylander, a former Washington Capital, signed a contract to play in Switzerland. William Nylander played for the Sodertalje and SDE HF junior teams and skated for Stockholm 1 in the TV-Pucken national tournament for high school players. He appeared in 12 U20 games for Sodertalje (including four playoff games) and scored 1 goal with 8 assists; finishing -9 with 4 penalty minutes. In 26 U18 games split between SDE and Sodertalje he scored 19 goals with 19 assists and was +14 with 16 penalty minutes. In eight TV-Pucken games for bronze medal-winning Stockholm 1 he scored 7 goals with 10 assists and was +15 with 14 penalty minutes.
2012-13: Nylander made his pro hockey debut in Sweden as a 16-year-old — skating for Sodertalje in Sweden's second league (Allsvenskan) — and was the leading scorer for the club's U20 team. He represented Sweden in a pair of international tournaments – returning to North America to play in the 2013 U17 World Hockey Challenge and concluding the season playing in the 2013 U18 World Junior Championship. Nylander scored 4 goals with 2 assists and was +2 with 2 penalty minutes in eight Allsvenskan regular season games and had a chance to skate on a line with his father at times. Sodertalje finished second but failed to earn a spot in the SHL after finishing sixth in the Kval Series playoffs. Nylander scored 2 goals with 1 assist and was -4 with 4 penalty minutes in 10 Kval games. In 27 games with the Sodertalje U20 team he scored 15 goals with 28 assists and was +14 with 14 penalty minutes. He scored 2 goals with 8 assists in six games as Sweden captured the gold medal in the WHC. Sweden's U18 team did not fare as well; finishing fifth in Sochi. Nylander scored 2 goals with 1 assist and was +2 with 2 penalty minutes in five games at the U18 WJC.
2013-14: Nylander represented Sweden in the 2013 U18 Ivan Hlinka Memorial Tournament in August before returning to Sweden. Signed to an SHL contract by MODO, he was loaned to Rogle and began the season in Allsvenskan. In four games at the Ivan Hlinka tournament he scored 4 goals with 2 assists and was +1 with 4 penalty minutes.