"When I heard there was a chance for City to land Kwanda, I was instantly excited,” McCarthy said.
Mngonyama's signing paved way for winger Aubrey Ngoma to join Sundowns, although the Tshwane giants had to part ways with a hefty amount of money to land their man.
Having spent the duration of the season on the sidelines, the 24-year-old is regarded as a promising defender by the legendary Bafana Bafana goal-scorer.
"Mngonyama is a soldier and probably one of the most promising defensive talents to come out of South Africa in a long time,” McCarthy said.
The KwaZulu Natal-born centre-back spent three years on loan at Maritzburg United, where he made 25 appearances in all competitions prior to his return to Sundowns.
"Seeing where the club is going, building for the future, this was a key acquisition for us. I can't wait to start working with the young man and get him to where he should be,” McCarthy said.
He could possibly make his debut when the Citizens travel to Rustenburg to take on Platinum Stars next weekend.