Karl Alzner was born on September 24, 1988 in Burnaby, British Columbia. A standout for the Calgary Hitmen of the WHL, Alzner's junior highlights include being named the WHL's top player and the CHL's top defenseman in 2008.
On the international stage, Alzner was part of the gold-medal winning Canadian teams at both the 2007 and 2008 World Junior Hockey Championships and was captain of the 2008 team.
A fifth overall draft pick of the Washington Capitals in 2007, Karl began his professional career in the American Hockey League with the Hershey Bears and when injuries created a hole in the lineup, Karl was summoned to make his NHL debut on November 26, 2008. After playing in 30 games for the Capitals, Alzner was returned to Hershey where he completed the 2008-09 season.
Alzner would go on to establish himself as one of the NHL's top defensive defenceman as a member of the Capitals. He would also prove to be one of the most durable players in the league. On January 27, 2016, Alzner became the Washington Capitals all-time franchise leader in consecutive games played. In a streak that began in the 2010-11 season, Alzer appeared in 423 consecutive games to surpass Bob Carpenter for the franchise record.