Nicknamed the "Condor", Johnson previously played for the St. Louis Blues, who selected him with the first overall pick in the 2006 NHL entry draft, as well as the Colorado Avalanche and the Buffalo Sabres.

Johnson was drafted from USA Hockey's National Team Development Program and played one season at the University of Minnesota before joining the NHL with St. Louis in 2007.

Johnson played hockey for the Academy of Holy Angels in Richfield, Minnesota, where he starred in his two seasons as a freshman and sophomore at the prep school.

As a 16-year old Johnson transferred to Ann Arbor, Michigan, to play for the National Team Development Program (NTDP).

Having been scouted as being tailored to suit the professional ranks with his hybrid two-way style and physicality, Johnson continued his rise in development the following season with a team-leading 49 points in 47 games with the under-18's.

[1] While also having a major impact at the junior international stage, Johnson was selected first overall in the 2006 NHL entry draft by the St. Louis Blues.

He joined a select group of five (now eight following Patrick Kane, Auston Matthews, and Jack Hughes) Americans to have been chosen first overall in an NHL entry draft, along with Rick DiPietro (2000), Bryan Berard (1995), Mike Modano (1988), and Brian Lawton (1983).

Johnson concluded his collegiate career after his freshman year when he was signed by the St. Louis Blues to a three-year, entry-level contract on April 20, 2007.

[17] In the first year of his newest contract, Johnson was named an assistant captain alongside David Backes, Barret Jackman, and Alexander Steen.

[13] Johnson began the 2011–12 season healthy but was shortly placed on injured reserve in November due to a groin injury.

[28] Upon returning to the lineup, Johnson recorded a career-high eight hits against the Dallas Stars, becoming the second Avalanche to reach this milestone since the league began tracking the stat in 2005.

[29] Prior to the conclusion of the 2013–14 offseason, Johnson was told by head coach Patrick Roy to come back with less bulk and less weight.

Four games into the 2020–21 season, Johnson sustained a concussion after a hit from Minnesota Wild forward Jordan Greenway caused his head to make contact with the ice.

He missed the remainder of the regular season, and while he returned to practice during the 2021 Stanley Cup playoffs, Johnson was unable to play before the Avalanche were eliminated in the second round by the Vegas Golden Knights.

[45] On March 8, 2024, with the struggling Sabres out of playoff contention, Johnson was traded to the Philadelphia Flyers in exchange for a 2024 fourth-round pick.

On July 1, 2024, after testing the market on the opening day of free agency, Johnson opted to return to the Flyers on a one-year, $1 million contract.

[49] As a product of the US NTDP, Johnson first made his international debut for the United States at the 2005 World U17 Hockey Challenge.

Representing the senior U.S. team at the 2010 Winter Olympics in Vancouver, Johnson received a silver medal after losing in overtime to Canada in the final.

[53] Johnson's uncle is Sean Duffy, the United States Secretary of Transportation, a Republican politician, and a former U.S. Representative for Wisconsin's 7th Congressional district.

[54] In addition to his accomplished career as an NHL defenseman, Johnson is a proprietor and commercial racehorse breeder for ERJ Racing, LLC.