The O’s confirmed the young central-defender had signed on a season long loan from Millwall on deadline day of the 2021 summer transfer window.
A tall and dominating centre-back, Mitchell, 19, signed for the Lions in 2018, before rising through the ranks, impressing in both the U18’s and U23’s.
This experience was furthered last season, when he joined National League side Bromley on loan in April.
Despite a late arrival to the team, Mitchell cemented himself as a starter, and impressed in his 11 appearances, which included a start in their narrow play-off quarter-final defeat to Hartlepool United.
Mitchell made his senior debut at The Den in their Carabao Cup win over Cambridge United, and is highly rated within in the Millwall set-up.