Mosimane confirmed the news of Johnson's appointment in a statement released on Monday afternoon.
Furthermore, the 56-year-old said the arrival of Johnson completes his technical team after he was allowed to bring with him three members of his backroom staff.
He was rumoured to be eyeing Rhulani Mokwena to assist him at Al Ahly but the young mentor decided to stay at Sundowns.
Johnson joins Kabelo Rangoaga and Musi Matlaba who left the Brazilians along with Mosimane.
"It gives me great pleasure to announce that coach Cavin Johnson has agreed to join me as an assistant coach at Al Ahly," said Mosimane.
"This completes the technical team I bring with me to the Red Devils after fitness trainer Kabelo Rangoaga and performance analyst Musi Matlaba," confirmed the 56-year-old.
Mosimane is confident Johnson's contribution will help the entire technical team deliver on its mandate.
"Coach Cavin is an accomplished and experienced mentor who has played an integral part in the development of football in South Africa and beyond.
He added: "With qualifications that include Caf Licence, Safa Pro Licence, Brazilian football academy, FA England courses and a diploma in Sports Management, he has what it takes to help us deliver on our mandate in this top league and beyond."
"The former head coach of Platinum Stars, SuperSport United, AmaZulu and also having served as the first assistant for Mamelodi Sundowns, head of youth development at Ajax Cape Town, was gracious enough to accept my request to uproot him from home to join me here."
Mosimane thanked the Al Ahly management for affording him the opportunity to bring in his own team.
"I want to thank the leadership of Al Ahly for allowing the opportunity to build the leadership that I believe can help us continue the winning culture of the club.
Johnson, 61, said it was an honour to be working with Mosimane at Al Ahly, and he thanked the Kagiso-born mentor for giving him the opportunity.
"It's an honour to be joining one of the most successful coaches in South Africa to coach the most successful club in Africa," said Johnson.
"I want to thank coach Pitso for giving this opportunity to work with this club of the century," added Johnson.
According to Mosimane's statement, Johnson will arrive in Cairo this week before officially being unveiled by Al Ahly.
Meet Cavin Johnson, my new Assistant Coach joining us at Al Ahly pic.twitter.com/1nJKrOTt54— Pitso Mosimane (@TheRealPitso) October 5, 2020