He played 24 seasons in the National Hockey League (NHL) for the New York Islanders, Ottawa Senators, Boston Bruins, and Washington Capitals between 1997 and 2022.

Internationally, he played for the Slovakia men's national team and won two silver medals at the Ice Hockey World Championships.

He is one of five European-born and raised captains to lead his team to the Stanley Cup championship, and the first to be born and trained in the Eastern Bloc.

[2] He would spend his first season in North America with the Western Hockey League (WHL)'s Prince George Cougars, who subsequently selected Chára in that year's CHL Import Draft.

His physical strength also gave him a very hard slap shot, which seemed to improve each and every season, although Chára was never deployed in an offensive role as an Islander.

Chára was traded to the Ottawa Senators during the 2001 NHL entry draft as part of a deal to acquire forward Alexei Yashin.

Chára was part of a package deal that included Bill Muckalt and the second overall pick from that year's draft, which the Senators used to select Jason Spezza.

In 2001–02, during his first year in Ottawa, he recorded new career highs in goals (10) and points (23) as he turned into a bona fide two-way defenceman.

[4] In 2003–04, Chára posted career bests with 16 goals and 41 points, and recorded the NHL's third highest plus-minus rating, behind Martin St. Louis and Marek Malík, culminating in his first James Norris Memorial Trophy nomination.

On 1 July 2006, the first day of the free agency period, Chára signed a five-year, $37.5 million contract with the Boston Bruins[5] and was named the team's captain, a role left vacant since Joe Thornton's departure to San Jose during the 2005–06 season.

Chára became only the third Slovak-born NHL player to become a team captain, after Peter Šťastný of the Quebec Nordiques and Stan Mikita (co-captain) of the Chicago Blackhawks.

Having raised $24,000 from the six competitors and their respective teams, the NHL and the National Hockey League Players Association (NHLPA), Chára set a new Skills Competition record with a 105.4 mph (169.7 km/h) slapshot.

He donated the winnings to Right to Play,[11] and due to Chára's leading role in fundraising for charity, the hardest shot competition that year was punningly dubbed a "Chara"ty event.

For his efforts, he won his first Norris Trophy as the NHL's best defenceman, edging Mike Green of the Washington Capitals and Nicklas Lidström of the Detroit Red Wings.

He also achieved a personal milestone that is exceptionally rare for a defenceman; on 17 January 2011, Chára recorded his first career hat-trick against the Carolina Hurricanes in a 7–0 victory.

[15] On 8 March 2011, Chára hit and drove Montreal Canadiens forward Max Pacioretty's head directly into an off-ice stanchion at the end of the bench, knocking him unconscious.

[17] For delivering the hit, Chára received a five-minute major penalty and a game misconduct, and videotape of the play was sent to NHL vice president of hockey operations Mike Murphy for review.

The following season the NHL tested and later mandated a change to its rinks to have a curved glass at the end of the player benches to deflect similar impact.

[22] In so doing, Chára became the first Slovak to captain a Stanley Cup champion, and only the second European to do so after Nicklas Lidström of Sweden for the Detroit Red Wings.

His point totals were down due to some of Boston's struggles down the stretch, particularly on the power play, where Chára only posted one assist all season despite plenty of ice time in man advantage situations.

[28] By the time the 2016–17 season had started, Chára was skating as part of the Bruins' top defensive pairing beside 6-foot-5-inch rookie defenceman Brandon Carlo, nearly 20 years younger than him.

[32] On 17 April 2019, in game 4 of the first round series between the Bruins and Toronto Maple Leafs, Chára became the oldest defenceman in NHL history to score a game-winning goal in the Stanley Cup playoffs.

[39] At the end of the season, he was a finalist for the Bill Masterton Memorial Trophy, awarded to the player who "best exemplifies the qualities of perseverance, sportsmanship and dedication to hockey".

In July 2008, he spent two weeks in Africa, visiting Mozambique in support of the charity, and climbing Mount Kilimanjaro with former NHL player Robyn Regehr.