The Dublin-born defender arrived at the Walkers Stadium in 2005 from Manchester City. McCarthy never played for City's first team but had enjoyed loan spells at Boston United and Notts County, who tried to sign him permanently, before the Foxes made their move. It didn't take him long to make a positive impression on City fans and coaching staff and he was named club captain for the 2006-07 season.
He became a bit of a fans' favourite due to his no-nonsense style of play but saw that first season cut short due to a shoulder injury in February 2007. Those games before his injury proved to be his last for the club, with McCarthy expressing a desire to leave the club before the start of next season. He was offered a new deal to stay but rejected it and promptly moved on to Charlton Athletic in the summer of 2007 for a fee of £650,000.
McCarthy was called for Republic of Ireland U21s and Republic of Ireland B and made 81 appearances for City in all, scoring four goals along the way.