Not only does the 22-year-old - along with his pal James McArthur - make frequent return visits to the club which gave them both their big break, the Lanarkshire club also have millions more monetary reasons to feel enhanced by their association with the player.
Although the exact details of the arrangement remain a closely guarded secret, Herald Sport understands that the club could net anything up to an additional £1.9m after McCarthy broke the transfer record for any Scottish-born player when he joined Everton from Wigan Athletic at approximately 10.55pm on Monday night. While the Goodison Park club said the total deal was worth £13m, and Wigan £15m, the rough calculation works out at about 20% of the profit Wigan took from the up-front £12m fee (less the £1.25m and the same in add-ons the club have already received).
While that cash will be priceless to a club which has already made a storming start to their SPFL Championship campaign and comes as a huge reward for all the development work done by many over the years, it wasn't just about the money last night. There was simply a joy and pride at the deserved progression of a player who was brought to the club by Billy Reid in the summer of 2006 and made his debut against Queen of the South on September 30 that year.
"It was time," Reid told Herald Sport last night. "James had served his apprenticeship there as well. He had proved that he is more than just a Championship player. You have to remember he is still only 22 years old, a baby yet. And I don't think it is the end of the road for him either. If he continues to want it as much as I know he does, I feel it is another stepping stone for James."
It was Reid who gave McCarthy his first big break, after the Castlemilk-born player had spent a season at Livingston. He wanted to move somewhere closer to home, with the catalyst being scout Willie Melville's decision to move from Livingston to Hamilton. "The first time I saw him playing, I got his mother and father in, and I told them I just couldn't believe the talent that he had," said Reid. "I thought he was an absolute stand-out. He was born to be a football player but it is not just the best guys who make it, it is how much they want it. What he did at Wigan is in the past, it is about what he does at Everton now."
Considering how obvious his abilities were at a young age, perhaps the most startling thing of all is that one man who didn't recognise his abilities was in charge of the SFA youth teams. Plenty other football managers have been damned by early snap judgments on players but there were scance looks all around from the Hamilton contingent when they were informed by Archie Knox that their 15-year-old first team player wasn't being considered for Scotland's under-17s, allegedly on the proviso that he wasn't able to get around the park. The Republic of Ireland weren't so reticent, however, and not soon afterwards McCarthy was opting to play for the country of his grandpa's birth instead.
It is an old story but one which only has added currency when you now consider that two of the top three all time 'Scottish' transfers didn't go on to play for the country. "I can remember it vividly, when I just shook my head in disbelief that they couldn't see this boy was a player," said Reid. "He was playing in our first team at 15 in the first division and we were in the top three, yet he wasn't deemed good enough to play for Scotland 17s. I am disappointed he doesn't play for Scotland too but that is the past."
"From our point of view it is tremendous and a wonderful selling point for everything that has been done at the club in the last 10 years," said the club's long-serving secretary Scott Struthers. "Whether it has been Billy Reid or Alex Neil, Ronnie MacDonald, Les Gray, Allan Maitland or myself, people don't appreciate the amount of hard work that goes on behind the scenes. We won division one in great style, losing only three goals at home, and James was a mainstay of that team as well as the team which stayed in the SPL. It was on the basis of that he was sold on to Wigan in 2009 but Liverpool, Reading, Deportivo la Coruna and Barcelona asked to be kept informed as well. All sorts of teams were in for him."
The man himself last night paid tribute to Martinez and vowed to live up to his massive transfer fee and the onerous billing of the most expensive Scots-born footballer of all time. "I worked under Roberto for four years at Wigan and I had four brilliant years," McCarthy said.
"I loved every minute. He has said I just need to come and work hard and that I'm not guaranteed to go into the team, so I'll give it my best shot. You see big transfers going all the time and obviously I'll need to put up with it. But, to be honest, I believe in my ability and I'll give it my best shot."
After the Republic's qualifiers against Sweden on Friday and Austria on Tuesday, he could make his debut next Saturday against Chelsea. At Hamilton, however, thoughts were already turning to how to spend the money.
"It just will allow us to invest in our infrastructure and allow us to get back some money that the directors have put in for the synthetic pitch etc," said chairman Gray. "And it will allow our academy to continue to flourish and develop young players - and young Scottish players at that. We may, if we are in a good position in January, decide to improve the squad. But we won't be throwing money around."