Born on 20 August 2005 in Kingston upon Thames, Leo Castledine grew up supporting Chelsea and began his footballing education in their academy before moving to AFC Wimbledon at the age of seven. At Wimbledon he transitioned from a defensive role into midfield, and in 2019 he became the youngest player in the club's history to make an under-18 appearance — doing so alongside players four years his senior. He returned to Chelsea in March 2020 and signed his first professional contract with the club on his seventeenth birthday in August 2022.
Castledine made his senior debut as a substitute in a 6–1 EFL Cup semi-final second-leg win over Middlesbrough in January 2024. He spent the 2024–25 season on loan at League One side Shrewsbury Town, before joining Huddersfield Town on a season-long loan in August 2025. His form at Huddersfield — five goals in his final month at the club — earned him both EFL League One Player of the Month and EFL Young Player of the Month for December 2025.
An attacking and technically assured central or wide midfielder, Castledine signed a permanent deal with EFL Championship side Middlesbrough in January 2026 on a four-and-a-half year contract, with the club viewing him as a key part of their long-term plans. At international level, he has represented England at under-15, under-17, under-18, and under-19 level, scoring on his England under-19 debut in a 6–0 win over Romania in November 2023. He is the son of former Wimbledon midfielder Stewart Castledine and television presenter Lucy Alexander.