Born in Oxford, forward Jack Johnson was inspired to start playing Rugby League when Rhinos legend Jamie Peacock told him to start playing on a rugby camp.

He went on to play for Peacock’s junior club Stanningley as well as West Leeds Eagles and later Leeds Irish, where he won the Leeds and District Cup final in 2019. He was part of the Rhinos' 2020 scholarship signings but didn't get any gametime as Covid saw the Under-16s season suspended. Coming back to action in 2021, he performed well for both the Under-16s and Under-17s at second row, making a total of five appearances and scoring three tries.

In October 2021, Johnson was one of seven players to pen their first academy contract with the club and in his first Academy season, he made eleven appearances and scored one try. The reintroduction of the Reserves competition in 2022 allowed the back rower to extend his game time as he made five more appearances, including starts in three consecutive wins over London Broncos, Newcastle Thunder and Huddersfield Giants.