The 18-year-old has signed a long-term deal with the Easter Road side but the Evening News understands he will initially spend time on loan with partner club Charleston Battery in the USL Championship.
Johnson, known as EJ, has been on the club’s radar since before Shaun Maloney’s arrival and featured as a trialist in a friendly win against Spartans in mid-October.
The Californian-born player can operate anywhere on the right side and started as a right-back in the 6-1 win at the Hibernian Training Centre.
Johnson has spent time with the Sacramento Republic academy and the BARÇA Residency Academy, an Arizona-based sporting and educational residential academy for young footballers in the US that has thrice been named the country’s best training facility by the US Soccer Federation.
The attacker joins João Baldé, Tom Carter, and Allan Delferrière as new additions to the Capital club's second-string side.
Academy director Steve Kean said last month that the club was putting the finishing touches to its development side for next season. The team will sit between the under-18s and the first team and will be used as a bridge between the academy and senior squad.
Kean explained: “The development team will be a group of lads who are aspiring to be in the first team but aren’t quite there yet.
“The team will compete in a lot of matches in England, against teams from the English U23 system, to help try and bridge the gap between under-18 football and first-team football, which I have said from day one is a huge jump."