He has previously played for the Los Angeles Kings, Pittsburgh Penguins, New York Rangers, Colorado Avalanche and Chicago Blackhawks.

Following two years with the USA Hockey National Team Development Program, Johnson was selected third overall by the Carolina Hurricanes in the 2005 NHL Entry Draft.

Johnson has represented the United States on the international stage multiple times, most notably playing for Team USA at the 2010 Winter Olympics in Vancouver.

[4] As a sophomore during the 2002–03 hockey season, Johnson scored 15 goals and 27 assists as he helped the school's Midget Major AAA team win the U.S. National Championship along with current NHL player and former teammate Sidney Crosby.

Johnson recorded his first NHL point on October 10, 2007, assisting a Kyle Calder goal in a loss to the Dallas Stars.

His play earned him a selection to the Team USA at the 2010 Olympics, along with Kings' captain Dustin Brown; the two eventually won the silver medal.

On February 23, 2012, Johnson was traded by the Kings to the Columbus Blue Jackets, along with a conditional first-round draft pick (Marko Daňo), in exchange for Jeff Carter.

Johnson added that he did not follow the Kings' subsequent run to win the Stanley Cup, as he was in Finland playing for Team USA at the 2012 IIHF World Championship.

[17] In the shortened 2012–13 season, Johnson was one of the Blue Jackets' alternate captains, and was considered the de facto leader of the team as Columbus nearly qualified for the 2013 playoffs.

[26] On January 13, 2018, it was reported that Johnson requested a trade from the Jackets, citing a reduced role as well as a desire to better position himself as a free agent in the summer.

[34] As an unrestricted free agent (UFA) heading into the 2021–22 NHL campaign, Johnson was invited to the Colorado Avalanche 2021 training camp on a professional tryout (PTO) contract on September 1, 2021.

[35] After participating in training camp and appearing in 5 out of 6 pre-season games with the Avalanche, Johnson made the club's final roster to open the 2021–22 season, securing a one-year, $750,000 contract, on October 10, 2021.

[38] After winning the Stanley Cup as a member of the Avalanche, Johnson left as a free agent to sign a one-year, $950,000 contract with the Chicago Blackhawks on August 17, 2022.

[39] Johnson made his Blackhawks debut on opening night of the 2022–23 season against the Avalanche, joining his former club in the raising of the Stanley Cup banner and appearing on ice for the ceremony on October 12, 2022.

[40] Deployed in an elevated top-four pairing role with the rebuilding Blackhawks, Johnson registered his first point with Chicago, assisting on a Sam Lafferty goal, in a 5-2 victory over the San Jose Sharks on October 15, 2022.

[44] A free agent at the end of the 2023–24 season, Johnson signed a one-year, $775,000 contract to return to the Columbus Blue Jackets on July 2, 2024.

[47] In 2008, he decided he no longer needed the guidance of long established sports agent Pat Brisson, and would rely on his mother, Tina Johnson, to manage him; in 2011, immediately before signing his first major contract – a seven-year, $30.5 million deal with the Los Angeles Kings – Johnson granted his mother power of attorney over all his finances, despite knowing, as he claimed in a later lawsuit against lenders, that she had no idea how to manage financial planning.

[49] Lenders, among whom was Iowa Congressman-elect Rod Blum, eventually brought three lawsuits against Johnson for defaulting on over $6 million in debt.