Johnson was born in Birmingham, and is a product of the West Bromwich Albion academy.
He joined Stevenage in 2013 and was their youngest ever debutant at 17-years and 215 days.
Johnson impressed enough to earn trials with a number of Premier League clubs, including Tottenham, Manchester United and Watford.
Standing at 6`2”, Johnson can play at both left-back and in central defence and also spent a period on loan with National League South side St Albans City and, earlier this season, with National League outfit Boreham Wood.
The 21-year-old defender, who has represented Northern Ireland at under-21 level, will play under former Stevenage favourites Dino Maamria and Jon Ashton and joins midfield team-mate James Ferry, who is already on loan at Liberty Way.