The former Platinum Stars coach has long been linked with United after the position became vacant, following the departure of Gavin Hunt three weeks ago to Bidvest Wits.
He signed three-year contract, following what was the worst kept secret in football.
“I’m happy to have joined United, and I believe we can achieve great things,” said Johnson.
“I’m up for the challenge.”
United chairman Khulu Sibiya said: “We are thrilled to welcome Cavin – one of the most exciting coaches in the country – while we continue with our policy of hiring local coaches.
Asked what was his mandate, Sibiya minced no words, and said he must do better than what he did at Platinum.
Today at PLS offices, Johnson will know what his new team is preparing for as the 2013/2014 PSL fixture season is unveiled.