Rangers boss Harry Redknapp has tipped McCarthy, who won three caps for England Under-21s, as a future full international, and was thrilled to have captured his services.
"I am delighted to bring Alex in," he said.
"He's a great young goalkeeper with a great future in front of him. I think he's got the potential to be a future England goalkeeper one day, so to have him here at QPR is fantastic.
"We've got Rob Green and Brian Murphy, and Alex's arrival here helps to further strengthen the goalkeeping department.
"I want to have competition for places in all areas of the pitch, and you can't have too much quality in your squad.
"He is a great signing for the club, and for the future of QPR."
McCarthy revealed the Redknapp factor was key in his decision to move to Loftus Road, despite late interest from a number of big top-flight clubs.
"I spoke to Harry a few times and that was probably the icing on the cake for me," he said.
"I'm only 24 and I want to be playing games - there are goalkeepers out there that have sat on the bench and been forgotten about. I don't want to be one of those. It's a great challenge for me."
And he is relishing the opportunity to return to the top level.
"I'm over the moon to be back in the Premier League," said McCarthy, who will come into contention after the international break.
"When you're growing up, all you dream about is playing in the best league in the world - I had a taste of it at Reading, but to be back here now is fantastic."
- Sports & Recreation