Johnson joined West Ham United when he was seven years old. Johnson began his childhood career as an attacking winger before transitioning to full-back. On December 3, 2017, he was named to the bench for West Ham’s 2-1 loss at Manchester City, and he signed his first professional contract when he turned 18 in January 2018.

Johnson made his Premier League debut for West Ham on February 27, 2019, starting at left-back in a 1-0 loss to champions Manchester City at the City of Manchester Stadium.[9] Johnson did not play again until 17 July 2020, when he appeared in West Ham’s 3-1 win against Watford due to two major hamstring injuries and the form of fellow defender Jeremy Ngakia.