The 24-year-old broke into the first-team at the Madejski Stadium during the club's 2012-13 Premier League campaign, earning himself a call up to the England national squad in May 2013, yet not featuring.
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And now McCarthy, who has also won three caps at U21 level, revealed his delight at returning to the Premier League after narrowly missing out on a return through a play-off place with Reading last season.
"I'm over the moon to be back in the Premier League," told the club's official website.
"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."
QPR manager Harry Redknapp was thrilled to capture the highly rated shot-stopper and confirmed he will vie with Rob Green for the number one spot along with added competition from Brian Murphy.
"He's a great young goalkeeper with a great future in front of him," Redknapp commented. "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."