McCarthy, who has reportedly joined for a fee which could rise to £2million, is regarded as one of the hottest prospects in British football and new Latics boss Roberto Martinez is pleased to have snapped him up for an undisclosed fee.
"There were several clubs chasing James," said Martinez. "It is fantastic he has chosen Wigan.
"He had a very difficult decision to make and, fair play to him, he took his time to weigh up all his options, take advice and make his mind up.
"That shows a huge amount of maturity for a boy of his age, but then maturity is probably the one word I would use above all others to describe him.
"He has the ability to become a big player in the future and it is great to have him on board."
Republic of Ireland Under-21 international McCarthy, who has signed a five-year deal at the JJB Stadium, made his debut for Hamilton aged just 15 and has gone on to make almost 100 league appearances.
Martinez added on wiganlatics.co.uk: "He's quick, very bright on the ball, can see a pass and I believe he can have a great career in football.
"Now it's up to us to manage those expectations carefully and nurture a really prodigious talent over the coming months and years.''
McCarthy joins Jason Scotland, Jordi Gomez and Hendry Thomas as new arrivals at Wigan ahead of the new Premier League season.