2008-09: William Karlsson skated in 33 games for the Arlanda Wings U18 team and scored 10 goals with 18 assists and 19 PMs.
2009-10: Karlsson made his debut in Sweden's U20 Super Elit league – playing six regular season games and two playoff games for Vasteras – after joining the club's U18 program. In 39 games of U18 play at the Allsvenskan (second) and Elit (third) league level, Karlsson scored 23 goals with 21assists and had 62 PMs.
2010-11: Karlsson was the leading scorer for Vasteras in the Super Elit U20 league and represented Sweden at the 2011 U18 World Junior Championship. Karlsson was the Super Elit league's fourth leading scorer with 20 goals and 34 assists in 38 games and was +28 though Vasteras missed the league playoffs. In five games for Sweden at the U18 tournament, Karlsson scored 2 goals with 3 assists, including an assist in the championship game against the USA. He finished +8 for the tournament and Sweden received the silver medal after falling to the Americans in overtime. Karlsson was ranked 21st amongst European skaters in Central Scouting's final rankings and was selected by Anaheim in the second round (53rd overall) of the 2011 NHL Draft.
2011-12: Karlsson was the second-leading scorer for Vasteras in Sweden's AllSvenskan (second league), trailing only former Yale star Broc Little, and played for gold medal-winning Sweden at the 2012 U20 World Junior Championship. He scored 12 goals with a team-high 33 assists and was plus-25 with 6 penalty minutes. Vasteras finished fourth in AllSvenskan during the regular season. In the four-team pre-Kval series, Karlsson scored 1 goal with 1 assist and was plus-one in six games. Vasteras finished fourth – failing to earn a chance for elevation to the Elitserien. In six games for Sweden at the WJC he was an even plus/minus with 1 goal and 1 assist.
2012-13: Karlsson made his debut in Sweden's Elitserien as a 19-year-old and was named the league's Rookie of the Year after his first season with HV71. He scored 4 goals with 24 assists in 50 games and was +17 with 12 penalty minutes. HV71 finished third in the regular season before losing to Linkopings in a playoff quarterfinals series. Karlsson had 2 assists and was -5 in the five game series. He skated for Sweden in the 2013 World Junior Championship and was +1 with 2 assists and 2 penalty minutes in six games. Sweden reached the gold medal game; falling 3-2 to the USA. Karlsson signed a three-year entry-level contract with Anaheim in May, 2013.
Karlsson has a speedy, two-way game that should prove suitable to the North American game as soon as he gains some more strength. Not only is he quick, he is also shifty and capable of making plays in traffic. Karlsson was outstanding for the Swedes in their sprint to gold at the World Juniors in 2011-12.
Karlsson made the jump from the second tier in Sweden to the top level last year and showed some signs of real quality. He signed a three-year entry-level contract with the Ducks in May of 2013 and looks like he will be heading over to North America in the next year or two.