After enjoying an excellent 2017 campaign in the MLS, Harrison was linked with a return to England having left for the United States aged 14.
Stoke City were allegedly keen on signing the England Under-21 international, but Harrison has instead penned a three-and-a-half-year deal at the Etihad Stadium.
Harrison will spend the remainder of the campaign at Championship side Middlesbrough, and the winger has admitted that he is excited to have been given an opportunity in England.
The 21-year-old told the club's official website: "I can't wait to get started over here. I've really enjoyed my time with New York City and I'd like to thank Patrick Vieira and his coaching staff for helping me to develop as a player.
"I feel I've really improved working under him and playing regularly in the MLS, but now I'm looking forward to testing myself in England.
"Middlesbrough are a good club, with Premier League experience and they're in the mix for a return via the playoffs this season. Hopefully I can help them achieve that."
Harrison leaves NYCFC having scored 14 goals in 59 appearances in the MLS.