Johnson succeeds Gavin Hunt, who left the club at the end of May and has since joined up with Wits.
Johnson was strongly linked with a move to Pretoria after impressing last season with Platinum Stars, eventually finishing runners-up to Kaizer Chiefs in the PSL.
Matsatsantsa have been without a coach since Gavin Hunt joined spending Bidvest Wits at the end of last season.
Johnson, whose mandate was to do better with SuperSport United than he had done with Stars, admitted he faced a tough task but said he was confident he had the right weapons in his arsenal to deliver.
"There's a lot of hard work ahead -- for myself and for the rest of the team," Johnson said.
"What I see, SuperSport United has an excellent group of players."
United chairman Khulu Sibiya said the club was proud to continue its policy of hiring local coaches.
"Cavin has a great history in the development of young players and has now shown his ability to coach at the highest level," Sibiya said.
"We know that he will help the club to continue our tremendous success."
SuperSport appoint Johnson
Johannesburg - SuperSport United have announced Cavin Johnson as the club’s new manager in a media conference on Wednesday afternoon.