City announced the signing of the 27-year-old on Twitter on Saturday morning, the day after transfer deadline.
The former Orlando Pirates hitman was picked up as a free agent, having walked out on Turkish outfit Genclerbirligi towards the end of 2017.
The Bafana Bafana international was also linked with SuperSport United and Mamelodi Sundowns, who on Friday confirmed the capture of fellow forward Phakamani Malambi from Egyptian giants Al Ahly.
McCarthy, Bafana's record marksman with 31 goals, has faith his former teammate can revive his career in the Mother City.
"We did hit a little bit of the jackpot because he is a player I am fond of," McCarthy said.
"He was a former teammate of mine. I know his capabilities when he is at his best, when he is fit.
"He hit a little bit of a rough patch but everybody does that in life. I don't think any person in the world, not even the great Richard Branson didn't also have to start from a rough patch in life.
"So I think 'TK' can be excused for that and that doesn't stop him being a world-class striker, one of the best strikers we had."
The Citizens coach is determined to get Rantie fit as soon as possible.
"That fact that now I have him, my main priority to get him fit as soon as possible because I know when he gets going he is unstoppable," added McCarthy.
The MTN8 finalists will resume their league campaign against Kaizer Chiefs at Cape Town Stadium on September 15 after the FIFA break.