Fast retired from the NHL on June 2, 2025. He missed the entire 2024-25 season after having neck surgery Aug. 6, 2024.

A native of Nassjo, Sweden, Fast was selected by the New York Rangers in the sixth round (No. 157) of the 2010 NHL Draft, but remained in Sweden through the 2012-13 season and joined the Rangers prior to 2013-14.

He made his NHL debut against the Phoenix Coyotes on Oct. 3, 2013, and played eight NHL games before being assigned to Hartford of the American Hockey League, where he set a team record for the fastest goal to start a period (seven seconds) against Springfield on Jan. 24, 2014.

Fast was called up to the Rangers and played the final three games of the regular season and three Stanley Cup Playoff games, getting an assist in his NHL postseason debut against the Philadelphia Flyers on April 17, 2014.

For the second consecutive season, Fast made the Rangers roster out of training camp in 2014-15, but was sent back to Hartford after three games. After scoring nine points (one goal, eight assists) in 11 games, he was recalled by the Rangers and remained with them for the rest of the season.

Fast scored his first NHL goal in a 5-2 victory against the Flyers on Nov. 29, 2014. After finishing with six goals and 14 points in 58 games, hwscored two goals in Game 3 of the 2015 Eastern Conference Final against the Tampa Bay Lightning to become the first Rangers rookie with a multigoal playoff game since Brandon Dubinsky in Game 3 of the 2008 Eastern Conference Quarterfinals against the New Jersey Devils and the first to score at least two goals in a road playoff game since Mike Ridley in Game 1 of the 1986 Patrick Division Semifinals against the Flyers.

Fast left the Rangers when he signed a three-year contract with the Carolina Hurricanes on Oct. 10, 2020. He signed a two-year, $4.8 million contract to stay with Carolina on July 1, 2023, in lieu of becoming an unrestricted free agent.