It was in the 2009 IIHF U-20 Championship where Karlsson made a name for himself. His efforts had him tied for first in scoring among blueliners with nine points and helped Sweden capture a silver medal.
What may be even more impressive was the fact that he beat out the likes of P.K. Subban, Ryan Ellis, and countryman Victor Hedman to earn the honors for top defenseman of the tournament.
Needless to say, his performance left Sens fans wanting more.
Though he was listed at 5'11" and weighed a mere 165 pounds, Senators management had seen enough of him to believe he could handle the responsibility of providing an offensive spark on defense.
There was no surprise that GM Bryan Murray moved up in the draft to take Karlsson with the 15th overall selection.
After Karlsson was signed to an entry-level contract at the conclusion of the 2009 regular season, Murray was ready to say that he expected him to crack the Senators top six defensemen. His play in training camp and during the exhibition schedule earned him a spot on the roster where he was instantly playing the majority of minutes on the powerplay.
Karlsson, who wears No. 65 to honor the birth year of both of his parents, struggled out of the gate.
After only nine games, he was sent down to the team's AHL affiliate in Binghamton, NY. At the time of his reassignment, he posted three assists but carried a minus-four rating.
While some would believe it could damage his confidence, Karlsson approached it as another challenge.
In his 12-game stint with the B-Sens, he collected 11 assists and earned a recall to Ottawa at the end of November. Since then, he has played in 35 games with the big club, registering another nine points. His first and only goal came on Dec. 19 against the Minnesota Wild.
Karlsson can attribute much of his success to two people in particular: Coach Cory Clouston and defense coach Luke Richardson. They have been working extensively to round off Karlsson's game, while he has also been given plenty of tips and advice from right-winger Daniel Alfredsson.
The Ottawa Senators captain has even opened his home to the young rookie in hopes to help him adapt to the North American lifestyle. It's a trend we've witnessed in the last few years for young players, as Sidney Crosby rooms with Mario Lemieux while John Tavares lives in Doug Weight's basement on Long Island.
Nevertheless, Karlsson's play has elevated over the last month. His contributions, both with and without the puck, have played a role in the Sens' 14 victories in their last 16 contests.
Many could say that the Swedish blueliner has luck on his side.
He suffered an upper body injury in a 3-1 victory over the Vancouver Canucks on Feb. 4 at Scotiabank Place and is currently on the injured reserve. The saving grace in this might be the timing, as he is able to use the Olympic break to rest up to ensure that he'll be able to play when the NHL resumes on March 2.
At this point, Karlsson has been left behind in hopes to win the Calder Trophy as the league's top rookie.
Fortunately, for the Senators, Karlsson's play has the potential to be a factor in a possible playoff run this spring.
Don't be surprised to hear his name more and more in the years to come.