The Barclays Premier League outfit edged past their Football League opponents 6-5 on penalties after Robins captain Gordon Greer blasted the final spot-kick of the night against a post.
It was tough luck on the Wiltshire outfit who battled hard throughout but remained in the tie due to Wolves' ineffectiveness in most areas through 120 goalless - and uninspiring - minutes.
But McCarthy knew it was going to be tough encounter and, as far he was concerned, it was job done - in the end.
"We had nothing more to achieve than to be in the third round," said the Wolves boss.
"We didn't do it in the way we would have liked, but we're in the hat."
The triumph represents the first time Wanderers, two-time League Cup winners, have made the third round since 2003.
McCarthy has seen his charges lose out to their fair share of lower-league sides in recent years and he knew they had nothing to gain and all to lose from the encounter.
"It was one of those games, I said to the players beforehand, 'you're damned if you do and you're damned if you don't'.
"All you can do is win it."
He added: "Overall, no great quality, there wasn't a great lot of chances, but we've won the game."
Robins winger Anthony McNamee saw his first-half strike bounce off the top of the crossbar, while team-mate Mark Marshall and Wolves striker Sam Vokes also saw tame efforts strike the woodwork.
Swindon goalkeeper David Lucas was forced to make some routine saves throughout but, in truth, neither side came seriously close to breaking the deadlock.
But the battling display provided by Swindon left manager Danny Wilson beaming.
"I'm both proud and disappointed," he said.
"We've come here and put in a fantastic performance.
"I think you only have to see the reaction of the fans after the game that suggests you've really given everything.
"They couldn't give us any more than they have here.
"They've come to a very, very tough place and put in a performance they can feel very proud of.
"The way the boys have played, throughout the side, they put so much effort and energy into the game.
"And they've been very unlucky not to win in normal time."
He added: "But we would have liked to have gone through, when you get so close, within touching distance, you're always disappointed to go out.
"But the players can feel very, very proud of themselves."