Johnstone will play for Macclesfield FC as their new number one goalkeeper.
The club achieved promotion to the NPL West Division this season, with the club working hard to bring in players to compete at that level.
A Macclesfield FC statement said: "Macclesfield FC are delighted to agree terms of shot-stopper Max Johnstone from Gulf United!
"The gifted goalkeeper, formerly of Fleetwood Town, Manchester United, Sunderland, St Johnstone & AFC Fylde is eager to impress between the posts for the Silkmen, signing from the Eagles for an undisclosed fee.
"Max, brother of Crystal Palace goalie Sam, moves to Macclesfield as the new number one, boasting a wealth of experience from the highest level of English football, and most recently playing a major role in Gulf's promotion to the UAE first division in his first season with the Dubai-based club.
"The 6ft5 24-year-old makes his way over from the Middle East to Macclesfield during the course of the summer."