Born in Watford, Grieves was first spotted at 8 years old when playing in a local tournament, and was signed in 2013 after a trial at the club.
[1] Jack was released from the Academy at 14 in 2019, and went to play football at Everett Rovers and then Leverstock Green.
[2][3] He made his senior Watford debut on 7 January 2023, coming on as a second-half substitute for Leandro Bacuna in a 2–0 home defeat to Reading in the second round of the FA Cup.
[4] The following month, he signed a three-year contract with the club holding an option to extend the deal by a further year.
[5] Grieves joined Ross County in July 2024 on a loan deal until the end of the season.