Billy Mitchell

Millwall

Mitchell hails from Orpington, but moved to Sidcup with his mother and brother when he was 15. He joined his boyhood club Millwall at under-15 level. His father's family originates from Bermondsey and are also Millwall fans, so is his maternal great-grandfather. He made his first team debut in the last game of the season on 5 May 2019, coming on a late substitute in the 1–0 defeat to Wigan Athletic. In the summer of 2019 he signed his first professional contract, a one-year deal. In April 2021, he signed a new long term deal with Millwall. He scored his first professional goal in a 4–1 win against Bristol City on 1 May 2021. Mitchell was awarded the PFA Championship Players in the Community award for the 2021–22 season after becoming heavily involved in Millwall's Community Trust.

Sheffield Wednesday

On 19 July 2026, Mitchell joined League One side Sheffield Wednesday on a three-year contract. He made his debut on 20 August 2026, starting the game in the 0–1 defeat to Bradford City.

As of 20 October 2026

Mitchell is the brother of Charlton Athletic player Zach Mitchell.