Mitchell started his youth career with Fletcher Moss Rangers; the same academy where teammates Cameron Borthwick-Jackson and Marcus Rashford also started their careers. He joined United in 2013, and was originally deployed as a winger for their youth teams. However, after Borthwick-Jackson was loaned to Wolverhampton Wanderers for the 2016–17 season, Mitchell was converted to a left-back in his absence. Mitchell was called up to the Manchester United senior squad on 7 May 2017, training with the first team ahead of a Premier League clash with rivals Arsenal, but was not named in the 18-man matchday squad. A week later, he was named among the substitutes in a 2–1 defeat by Tottenham Hotspur, but again did not feature. He made his senior debut in the final game of the season, starting against Crystal Palace. The following season, Mitchell played in thirteen Premier League 2 fixtures, before going out on loan to Hearts.
Loan to Heart of Midlothian
On 11 January 2018, Mitchell was loaned to Scottish Premiership side Heart of Midlothian until the end of the season. He joined up with the squad at their winter training camp in Valencia. Mitchell made his debut for Hearts on 21 January 2018, playing from the start in a 1–0 win over Edinburgh Derby rivals Hibernian at Tynecastle Park, in the fourth round of the Scottish Cup, winning the man of the match award. Mitchell scored the first senior goal of his career in Hearts' subsequent Scottish Cup tie, a 3–0 fifth round home win over St Johnstone. In March, Mitchell returned to United for treatment for an injury to his meniscus tendon and was ruled out until the end of the season. He had first picked up the injury during a match against Ross County in February. Ultimately, he returned to training earlier than expected, playing in two further matches. In all, Mitchell made 11 appearances for Hearts in this spell, scoring once in all competitions.
Second loan to Heart of Midlothian
Mitchell returned to United for their preseason tour of America in July 2018, and subsequently signed a new contract with the club until 2020. On 28 August 2018, Mitchell was once again loaned to Hearts, this time on a season long loan. On his return, Mitchell stated that he felt he had "unfinished business" at Hearts due to his previous injury. In February 2019, he suffered a recurrence of the injury in a Scottish Cup game against Auchinleck Talbot, and was ruled out for the rest of the season after undergoing an operation.
Recovery from injury
Mitchell returned to training at Manchester United in September 2019, as he continued his recovery from the injury. In June 2020, Manchester United announced that Mitchell would be leaving the club upon the expiration of his contract.
