The 25-year-old was the overwhelming favourite going into the fight against the Pennsylvania southpaw, and was eager to impress in his first fight on home turf in over four years.
And he did so in superb fashion, taking control of the bout from the first bell to the last, not losing a round as all three judges scored the fight 120-108 in his favour.
Karpency, who battled gamely throughout, found his opponent's speed and accuracy too much to handle but refused to be bowed despite the severe punishment he took and he ended up going the distance thanks to his sheer bloody-mindedness.
"It was good boxing match. It was an excellent fight," he said. "Tommy showed tonight why he has sparred with world-class fighters. He is very tough, has good power and is a tough fighter.
"A lot of people criticised our choice of opponent but I have moved through the British scene with ease, I was in university when I was knocking out the British guys.
"So it shows at world level you can't take anything for granted. I knew if I took my eye off the game he would punish me.
"He was probably the toughest opponent I have faced over the distance. A lot of guys are tough over six rounds, but wilt in the championship rounds.
"But Tommy is a 12-round fighter and it makes a difference."
And Cleverly admitted his dominant performance had been the perfect manner in which to mark his return to Wales.
The win also marked stage two of a potential treble of sporting success for the principality following the rugby team's Triple Crown-clinching win over England at Twickenham on Saturday, with Cardiff facing Liverpool in the Carling Cup final on Sunday.
He said: "I was pleased with the homecoming, that was the important thing about tonight, bringing world title boxing back to Wales and it was a special occasion and to win every round is great."
Karpency, sporting the signs of the battering he had taken, was equally fulsome in his praise of Cleverly and believes the Welshman is capable of taking on and beating anyone in the light-heavyweight division.
Karpency, who has sparred with the likes of Roy Jones Jr, said: "There is nobody who I have been in with who could knock over Nathan.
"Having gone 12 rounds with him tonight I say he could fight with anyone in the world.
"He is in top condition, he has a high work-rate and he takes good shots. I hit him with some real good shots tonight and he just sucked them up which surprised me more than anything.
"He is ready for anyone."
The mutual respect between the two fighters was in stark contrast to the scenes in Munich last weekend, where Dereck Chisora and David Haye clashed at a press conference after the former's defeat to Vitali Klitschko.
And Cleverly added: "That was a disappointing day for British boxing - and because it's boxing, it's magnified.
"It was about remaining professional today and doing the talking in the ring.
"The hype in the press conferences, everybody loves it - I love it, the public love it - but you've got to draw the line and fortunately tonight, no events like that happened."