[4] He made his debut for the club's reserve team at the age of 16, coming on as a substitute for Tom Lawrence in a 4–1 win over Bolton Wanderers on 16 September 2013.

[5] In July 2014, Borthwick-Jackson played in all five of the under-17s' matches at the Milk Cup, scoring in both the 4–0 semi-final win over Scottish side Partick Thistle[6] and the only goal in the final against French club Vendée.

[7] He held a regular spot in the under-18s throughout the 2014–15 season, playing in 28 of their 29 league games, and scoring the second goal in their 2–1 win over Blackburn Rovers on 31 January 2015.

[9] He made the step up to the club's under-21s in 2015–16, and on 7 November 2015, he was named on the first-team substitutes' bench for the first time against West Bromwich Albion; he came on for Marcos Rojo in the 76th minute for his senior debut.

[20][21] The lack of game time led to Borthwick-Jackson returning to Manchester United to play for the Under-23s in the second half of the season after a mutual agreement between the two clubs was made.

[32] On 7 June 2021, it was announced that Borthwick-Jackson was joining League One club Burton Albion on a two-year deal following the expiration of his contract at Oldham.

[33] On 26 July 2023, Borthwick-Jackson joined Polish Ekstraklasa side Śląsk Wrocław on a two-year deal with an optional one-year extension.