Sterne, who also won last week's Alfred Dunhill championship, reached the playoff by shooting a bogey-free 6-under-par 66 after beginning the round locked in a tie for 11th place.
He finished his dramatic comeback by making birdies at the 17th and 18th.
Both players finished regulation with totals of 14-under 274.
Maybin (70) was tied for second when the round began, but ultimately, bogeys at the fourth and eighth proved costly.
"This tournament is so prestigious and the names on the trophy are just unbelievable," Sterne said after becoming the third straight South Africa native to win the event at Paarl's Pearl Valley estates. "I may need to set a few new goals after this one."
Since 1903, only nine players born outside South Africa have won this tournament.
At 13-under 275, Ernie Els (64), Rory McIlroy (70) and Lee Westwood (73) shared third place.
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