Sterne fired a 6-under 65 in the final round at Royal Johannesburg, posting four birdies in his last eight holes, including two in a row at the end, to make
Mulroy and Carlsson made three birdies apiece on the back nine to shoot matching 5-under 66s and force the playoff. Like Sterne, Carlsson finished with back-to-back birdies at the 17th and 18th holes.
Darren Clarke shot a 68 and finished two shots out of the playoff at 11-under 273.
Sterne made birdie from a fairway bunker at the first playoff hole, the par-5 18th, after knocking a stiff 4-iron from 215 yards to 25 feet. It was his best shot of the tournament.
'They aren't my favorite shots, those long bunker shots. I just hit it as hard as I could and it came out absolutely perfect,' said Sterne.
But Carlsson and Mulroy also made birdies there, with Carlsson blasting out from a greenside bunker to two feet and Mulroy rolling in a 10-foot putt.
Back at the 18th again for the second extra hole, Sterne had the immediate advantage as the only one of the three players who didn't hit into a bunker off the tee.
Carlsson knocked his sand shot into the rough, while Mulroy only managed to blast his midway up the fairway. Meanwhile, Sterne reached the green in two shots again, sticking his ball within 10 feet.
'It's nice under that pressure to hit those shots at that time,' Sterne said of his playoff approaches. 'It gives me a lot of confidence and hopefully that will come in handy in bigger events.'
Carlsson chipped to 18 feet and rolled his birdie try past the right side of the cup. Sterne then came up short on his eagle putt and tapped in for a birdie, putting the pressure on Mulroy.
Faced with a 4-footer and a chance to force another playoff hole, Mulroy lipped out his putt to take par and give Sterne the win.
It was the South African's third career European Tour title following victories at the 2004 Madrid Open and last June's Celtic Manor Wales Open.
It was also the third playoff the first seven events of the 2008 season, which began in November with Phil Mickelson beating Ross Fisher and Lee Westwood in extra holes at the HSBC Champions Tournament.
'It was quite enjoyable, actually,' Sterne said of the pressure-filled playoff.
He was fortunate to be there at all after being as high as 3 over par during the first round on Thursday. His opening score of 71 was six shots off the first-round lead.
Sterne battled back with a 68 on Friday to make the cut, then shot a 67 in the third round Saturday to move into a share of 11th place overnight. His final-round 65 was the best score on Sunday.
'After the start I was quite far back and I knew I needed to get to 13 or 14 under,' Sterne said. 'I hit a couple of great shots [in the playoff] ... and here I am.'
Mark Murless closed with a 2-over 73 and finished in a three-way tie for fifth place at 9-under 275 after holding the lead after the second and third rounds.
Louis Moolman (69) and Chris Williams (70) also finished at 275.
Tyrone Ferreira (72) was alone in eighth place at 8-under 276, while 2007 champion Ariel Canete (67) led a group of five players at 277.