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Johnson, penned a three-year deal with the Tshwane outfit, ending speculation over the vacant coaching post.
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.
And though there are still kinks to be ironed out with Stars, who still maintain he is their coach, United insisted they have done everything above board and that Johnson will be having his first training with the team on Monday.
"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."