Johnson has put pen to paper on a four-year deal for an undisclosed fee believed to be around £6million.
McCarthy tried to sign Johnson from Cardiff during the summer of 2009, but he opted to switch to St. Andrew's in a £5million deal.
He said: "I saw something in Roger a couple of years ago and thank you for your patience is what he said! Two years is a bit too long to wait really.
"But we didn't get him the last time. He was my first choice then and we couldn't get him.
"He was the first choice this time and it has worked out for us."
McCarthy is impressed with how Johnson switched effectively from the Championship to the Premier League with the Carling Cup holders.
He said: "We've had a lot of players step up from the Championship as well and you just don't know when they come up.
"I always thought he would be a success in the Premier League. He was better than anyone thought.
"Fair play to Alex McLeish, who took him to Birmingham and he has had two great years. I'm delighted we finally signed him."
Johnson admits the lure of staying in the Premier League led to him leaving St Andrew's.
He said: "I made some great friends there. It was a difficult decision but I want to be in the Premier League, that's the end of it.
"I'm sure a few people will be disappointed I've not stuck it out, but I'm 28 and I want to be in the Premier League for as long as I can.
"Nothing against Birmingham, I had some great times there, the fans were great for me. But for me this feels right."
Johnson added: "Some fans will understand why I've moved, that is an opportunity to progress again, while some will be really disappointed I've jumped ship.
"But I reiterate again, I want to be in the Premier League for as long as I can.
"It's been a long road to get here and I don't want it to end."