Crushed 6&5 by his fellow American in last year's quarters, Kuchar captured his first world championship by a 2&1 margin.
Ian Poulter, meanwhile, had to settle for fourth place, but is at least back in the world's top 10 for the first time in 25 months.
Europe's Ryder Cup hero failed to claim the trophy for the second time in four years after losing 4&3 to Mahan in the semi-finals, then went down to a last-green loss at the hands of Australian Jason Day.
Mahan had not trailed all week until he bogeyed the fourth hole of the final, but by the turn was four down.
Kuchar lost the 10th and 11th before holing what appeared a crucial 12-foot birdie putt at the next after Mahan had fired in his tee shot to four feet.
Kuchar also birdied the long 13th, only for Mahan to strike back in kind on the next.
They shared the driveable 15th in birdies, but Kuchar fired his tee shot to the short 16th into the grandstand and was only one up.
Both bunkered their drives off the next, but Mahan found the worse lie and could not recover.
Poulter had been trying to make it back-to-back World Golf Championship victories after his triumph at the HSBC Champions in Shanghai last November.
He had beaten Stephen Gallacher, Bo Van Pelt, Tim Clark and Steve Stricker to reach the last four, but an inspired 20-yard chip-in by Mahan from over the short 12th proved decisive.
"What an incredible feeling to be standing here after six matches," admitted Kuchar. "When these dreams come true it's a special feeling.
"I was trying to do the same that Hunter did to me, but he played some great golf coming in and has a lot to be proud of."