At age 14, Jack made the decision to leave the Manchester United academy to attend Berkshire School in Sheffield, Massachusetts,[5] also representing their affiliated club team, Black Rock FC.
[8] Harrison signed a National Letter of Intent to play college soccer for Wake Forest University in Winston-Salem, North Carolina.
During the 2015 NCAA Division I men's soccer season, he made 22 appearances with Wake Forest, scoring eight goals, and providing 11 assists.
[12][13][14] Shortly after the draft it was discovered that Harrison had a fractured pelvic bone which left him on the sidelines for the first three months of his New York City career.
[15] Harrison made his professional debut on 21 May 2016 as a 57th-minute substitute for Thomas McNamara in a 0–7 loss against New York Red Bulls at Yankee Stadium.
[18] On 3 July, Harrison scored the opener in the Hudson River Derby in a man of the match performance as New York City recorded their first win over the Red Bulls.
[19] At the end of the season, Harrison was nominated for, but ultimately did not win, the MLS Rookie of the Year Award, which went to Jordan Morris of the Seattle Sounders FC.
[33] He made 4 appearances for Middlesbrough in total, being unable to displace the regular wingers Adama Traoré and Stewart Downing.
[39] He made his debut for Leeds in the opening game of the season on 5 August, as a substitute against his hometown club Stoke City at Elland Road in a 3–1 win.
[43] He scored his second goal of the season on 11 January 2019 against his former New York teammate Frank Lampard's side Derby County in a 2–0 victory.
[44] In his 100th professional fixture, Harrison scored the winner in a 1–0 win (his fourth goal for 2018/19) over Sheffield Wednesday at Elland Road,[45] the second game of the season[46] in which he claimed the match-winner.
[55] On 10 August 2020, it was announced that Harrison would join Leeds for a third successive season on loan,[56] this time with a view to a permanent move, in preparation for the club's return to the Premier League.
On 2 July 2021, Leeds officially announced the permanent signing of Harrison on a three-year contract for a transfer fee of £11 million.
[63] On 16 January 2022, Harrison scored his first senior hat-trick, helping Leeds to a 3–2 victory over West Ham United in the league.
[67] On 14 August 2023, Harrison returned to the Premier League, signing on a season-long loan to Everton, beating Aston Villa to his signature.
[72][73] On 1 October 2017, Harrison was called into the England U21 squad for the first time after injuries to Ruben Loftus-Cheek and Sheyi Ojo for games against Scotland and Andorra.
[77] Former New York City teammate David Villa described Harrison, "I've spent a lot of years in this game, and he has something important that only a few players have, when he has the ball at his feet, you get the sensation something special is going to happen.