Spieth carded a par on the fifth extra hole while Hearn and Johnson each bogeyed, earning him the first PGA Tour win for a teenager since Ralph Guldahl won the 1931 Santa Monica Open.
He charged into a share of lead by notching birdies on five of the last six regulation holes, including a dramatic chip-in shot from a bunker on the 72nd hole at TPC Deere Run.
That effort gave him a third consecutive round of 6-under 65, matching Johnson (68) and Hearn (69) at 19-under-par 265.
Martin Flores turned in a sterling round of 8-under 63 to pull into a share of third place with third-round leader Daniel Summerhays (72) and Jerry Kelly (68) at 18-under.
2014: NFL Cheerleaders [PHOTOS]
Michael Sam cut from the Dallas Cowboys