McDonagh made a big impact with his all-around game long before he reached the NHL, and he's continued to make an impact with the New York Rangers and Tampa Bay Lightning. He helped the Rangers make the Stanley Cup Final in 2014 and was a top-four defenseman for the Lightning when they won the Cup in 2020. He was traded to the Nashville Predators by the Lightning on July 3, 2022. He waived his no-movement clause one season after helping Tampa Bay win back-to-back Stanley Cup championships.

McDonagh was reacquired by the Lightning from the Predators on May 21, 2024, for a seventh-round pick in the 2024 NHL Draft and a second-round pick in the 2025 NHL Draft. The Lightning also received the Edmonton Oilers' fourth-round pick in 2024. He had two assists in his 1,000th NHL game, Tampa Bay's 8-0 win against the Utah Hockey Club on March 27, 2025.

McDonagh signed a three-year, $12.3 million contract (average annual value of $4.1 million) with the Lightning on Dec. 4, 2025.

The nephew of former NFL and University of Miami quarterback Steve Walsh started to stand out in high school with Cretin-Derham Hall in St. Paul, Minnesota. As a junior in 2006, he helped lead Cretin-Derham to its first state championship with 12 goals and 45 points. As a senior in 2007, McDonagh was named Mr. Hockey for the state of Minnesota after finishing with 14 goals and 40 points in 26 games.

The Montreal Canadiens selected McDonagh with the No. 12 pick in the 2007 NHL Draft before he headed to the University of Wisconsin. As a freshman, McDonagh was named to the 2008 Western Collegiate Hockey Association All-Rookie Team, and in 2009 he was named to the Academic All-Big Ten and WCHA All-Academic teams.

Between his sophomore and junior seasons, McDonagh was traded to the Rangers on June 30, 2009. As a junior, McDonagh scored four goals and 18 points to help the Badgers reach the 2010 NCAA Frozen Four championship game, where they lost to Boston College.

McDonagh left Wisconsin after that season, signed an entry-level contract with the Rangers on July 6, 2010, and was assigned to Connecticut of the American Hockey League. After 38 games, one goal and eight points, McDonagh was called up and made his NHL debut against the Dallas Stars on Jan. 7, 2011.

He remained with the Rangers for the rest of the season, appearing in 40 games and scoring his first NHL goal in the regular-season finale on April 9, 2011, a 5-2 victory against the New Jersey Devils. In his second season, McDonagh was a mainstay on the New York defense, scoring seven goals and 32 points and averaging 24:44 of ice time in 82 games.

The Rangers signed McDonagh to a six-year contract extension on July 8, 2013, and he responded with a 14-goal, 43-point season in 2013-14. He tied for fifth in the League with three shorthanded goals and was eighth in voting for the Norris Trophy. With a six-game point streak from Nov. 27-Dec. 7, 2013, McDonagh became the first Rangers defenseman to get a point in at least six consecutive games since Brian Leetch in 2003-04. His six goals in March were the most by a New York defenseman in a calendar month since Leetch in January 1998.

During the Rangers' run to the 2014 Stanley Cup Final, McDonagh set a franchise record in the Eastern Conference Final when he became the first defenseman to have eight assists in a playoff series and the second to score 10 points in one playoff series, accomplished first by Leetch (11 points) in the 1994 Stanley Cup Final. His four points in Game 1 of the Eastern Conference Final matched a Rangers single-game playoff record for a defenseman, accomplished twice by Leetch and once each by Dave Maloney and Brad Park.

McDonagh finished the 2014 postseason with four goals and 17 points in 25 games. In 2014-15, he finished with a plus-23 rating, fifth among NHL defenseman. During the 2015 playoffs, McDonagh became the first Rangers defenseman to score an overtime goal with his team facing elimination when he scored 9:37 into OT in Game 5 against the Washington Capitals on May 8, 2015.

On Oct. 6, 2014, he was named the 27th captain (and fourth-youngest) in Rangers history. He remained captain until Feb. 26, 2018, when he was traded to the Lightning, who signed him to a six-year contract July 1, 2018.