[6][3][7] Fox grew up playing for the Long Island Gulls of the Atlantic Youth Hockey League.

[8] In his freshman season, Fox helped Harvard win the Beanpot for the first time since 1993 by recording a goal and an assist to beat Boston University 6–3.

[13] Fox also helped Harvard reach its first Frozen Four series since 1994 and win an Ivy title and ECAC Hockey Championship.

[3] Fox broke the school record for points by a Harvard defenseman in one season, set by Mark Fusco in 1983.

[16][2][17] On June 23, 2018, the Calgary Flames traded Fox's NHL playing rights (along with Dougie Hamilton and Micheal Ferland) to the Carolina Hurricanes in exchange for Elias Lindholm and Noah Hanifin.

[18][19] Flames general manager Brad Treliving had expressed doubts about the team's chances of signing him.

[20] The likelihood that Fox would sign with the Hurricanes diminished with the team becoming a playoff contender with a surplus of quality defensemen and a clause in the NHL's collective bargaining agreement (CBA) allowing an NCAA player to return for his senior year and become a free agent upon graduation.

[29] He became the fifth rookie defenseman in franchise history to have at least 40 points in a season, joining Ron Greschner, Reijo Ruotsalainen, Brian Leetch, and Mike McEwen.

During the 2020–21 season, on April 6, during a 8–4 win over the Pittsburgh Penguins, Fox became the fourth defenseman in Rangers history to record an 11-game point streak.

He is the youngest winner since then Montreal Canadiens defenseman P. K. Subban won it at the age of 23 in 2013, and is the first Rangers player to receive the honor since Brian Leetch in 1997.

[36][37] In a shortened 56-game schedule, Fox led all defensemen with 42 assists and was second in points with 47, behind Tyson Barrie of the Edmonton Oilers.

[38][39] In January 2022, Fox was chosen to his first NHL All-Star Game, which was held at T-Mobile Arena in Paradise, Nevada.

The following year, Fox was named an alternate captain for Team USA at the 2018 World Junior Ice Hockey Championships.

[44] Following the conclusion of his collegiate career, on April 19, 2019, Fox was selected to represent the senior United States team at the 2019 IIHF World Championship held in Bratislava and Košice, Slovakia.