McDonagh attended Cretin-Derham Hall High School in Saint Paul, Minnesota, where as a junior he helped lead his team to the school's first state hockey championship. The following season, he was awarded the 2007 Minnesota Mr. Hockey award, which is given annually to the top senior high school hockey player in the state.[1]
On July 6, 2010, McDonagh signed an entry-level contract with the Rangers, forgoing his senior season at Wisconsin and joining childhood friend and former Wisconsin teammate Derek Stepan in New York.[6][7]
On April 9, 2011, McDonagh scored the game-winning goal, the first of his career, against the New Jersey Devils, earning the Rangers the eighth and final playoff spot in the Eastern Conference.[14]
On July 8, 2013, McDonagh signed a new six-year, $28.2 million contract with New York.[16] He led all Rangers defensemen with 43 points in the 2013–14 season as the Rangers finished fifth in the Eastern Conference.[17] The team defeated the Philadelphia Flyers, Pittsburgh Penguins and Montreal Canadiens en route to the 2014 Stanley Cup Finals against the Los Angeles Kings, where the Rangers lost the series 4–1. During the playoffs, McDonagh was tied for the lead amongst defensemen in assists (13) and ranked second in points (17). For his season's efforts, McDonagh was named Rangers' MVP and the recipient of the Players' Player Award, given to the Ranger "who best exemplifies what it means to be a team player". He became just the second Rangers defenseman to hold both awards after Brian Leetch did so in the 2002–03 season.[18]
On October 6, 2014, McDonagh was named the 27th captain in Rangers history, filling the vacancy left by Ryan Callahan the previous season,[19] and the first defenseman since Brian Leetch in 1997 to be named captain. Martin St. Louis, Marc Staal, Dan Girardi and Derek Stepan were named his alternates. McDonagh became the fourth-youngest Ranger captain and 11th defenseman to hold the honor. In his first season as captain, he led his team to the Presidents' Trophy and were ultimately eliminated in the Eastern Conference Finals by the Tampa Bay Lightning in seven games.[20] On May 30, 2015, it was revealed that McDonagh had played the final three games of the series with a broken foot.[21]
McDonagh continued his strong play and leadership into the 2015–16 season, his second as team captain. However, during a February 6, 2016, game against the Philadelphia Flyers, McDonagh suffered a concussion after getting punched by Flyers' forward Wayne Simmonds after McDonagh cross-checked Simmonds.[22] McDonagh missed eight games as a result, including a February 14 rematch between the two teams, during which Rangers' rookie defenseman Dylan McIlrath fought Simmonds.[23] Still feeling the effects of the concussion, McDonagh was scratched for two games of the Rangers first round playoff series against the Pittsburgh Penguins, which the Rangers ultimately lost in five games as the Penguins eventually went on to win the Stanley Cup.
On June 1, 2018, Ryan McDonagh signed a seven-year, $47.25 million contract extension with the Lightning. During the 2018–19 season, McDonagh would enjoy the best offensive season of his career, playing all 82 games and scoring 9 goals and 46 points during his first full-season in Tampa Bay, finishing 8th in Norris Trophy voting. However, McDonagh was held pointless in Tampa Bay's first-round upset loss to the Columbus Blue Jackets.
McDonagh continued his role as one of Tampa Bay's shutdown defenders in the COVID-delayed2020–21 season. On June 8, 2021, McDonagh played in his 150th Stanley Cup playoff game against the Carolina Hurricanes. The Lightning would go on to repeat as champions in 2021, winning McDonagh his second-consecutive championship. McDonagh led all skaters in plus–minus at +18 during the 2021 Stanley Cup playoffs.
After two seasons in Nashville, McDonagh was re-acquired by the Lightning on May 21, 2024, in exchange for a second-round pick in 2025 and a seventh-round pick in 2024.[27]
After being drafted, McDonagh attended the University of Wisconsin–Madison for three seasons, playing with their ice hockey team, before foregoing his final year to turn pro with the Rangers. In January 2021, McDonagh re-enrolled with the school, taking online classes to complete his final 18 credits and graduate with a degree in Personal Finance.[30] McDonagh studies on days he does not have NHL games and revealed he had always planned to return to school to graduate but core courses were not available remotely while he played professionally, and was able to attend again after courses he was interested in were migrated online during the school's response to the COVID-19 pandemic in Wisconsin.[30]