A native of Natick, Massachusetts, John was selected 27th overall in the 2008 NHL Entry Draft by the Washington Capitals. He made his NHL debut for the Capitals in 2009 and is recognized as one of the NHL's top young defensemen.
Internationally, Carlson was an alternate captain for the gold-winning US team in the 2010 U20 World Junior Hockey Championships. He was named to NHL.com's All-Time World Junior Championship team. John also represented his country as a member of the USA Men's Hockey Team at the 2014 Olympic Games in Sochi and the 2016 World Cup of Hockey in Toronto.