He didn't have to. Thanks to the gallery, he heard it.
"I figured it was about 10 feet," Driscoll said.
The crowd got even louder when Johnson sank the birdie putt to successfully defend his Texas Open title and end Driscoll's unlikely final-round surge from eight strokes back at La Cantera Golf Club.
Johnson's win capped a wild shootout in which seven players stood within a stroke with four holes left. The 2007 Masters champion followed his third-round 60 with a 70 to match Driscoll at 15-under 265, then hit the 6-iron approach in the playoff to set up his sixth career PGA Tour victory.
"I feel very lucky," said Johnson, who earned $1,098,000.
Johnson vaulted to the top of the FedEx Cup standings, passing Geoff Ogilvy and Phil Mickelson -- the only other two-time winners on the PGA Tour this season. Johnson also won the Sony Open in January.
Paul Goydos had a one-stroke lead with two holes to play, but closed with two bogeys for a 69, leaving him a stroke back along with Bill Haas (65), who birdied five of six holes on Nos. 11 through 16 but missed a 6-foot putt on the par-3 17th.
Driscoll was an afterthought at 7 under when the final round began, eight strokes behind Johnson and his group that included Goydos and Leonard. After wrapping up his 62, Driscoll had to wait more than an hour for Johnson to finish his round.
It was worth the wait, but it didn't last long.
"You want to put the pressure on your opponent. There's no doubt about it," Johnson said. "It's not like he hit a bad shot. He hit 20-plus feet. He hit a good putt. So it just kind of went my way."
Driscoll was in position for the biggest comeback in the history of the Texas Open, which dates to 1922 and is the third-oldest event on the PGA Tour. Instead, Driscoll settled for only his third top-10 finish and first since a runner-up effort in the 2005 Zurich Classic of New Orleans.
"When you get that close to a win and that close to going to Augusta and going to Hawaii for the first tournament of the year, it's a little disappointing," said Driscoll, who has conditional status on the tour after finishing 141st on the money list last year.
Goydos briefly topped a crowded leaderboard with a 13-foot birdie putt on the par-4 16th before falling back on the last two holes, putting the 44-year-old and sympathetic favorite out of the running.
Goydos was playing four months to the day his former wife died. His final flaw, muffing his chip shot to a measly 7 feet on 18, denied him a spot in the playoff.
Goydos, who started the round two strokes back, led after 36 and was eyeing his first tour victory since 2007.
"For 70 holes I did really good," Goydos said. "Didn't hit a good putt on 17 and didn't hit a good shot on 18. And guys who play like that will do it."
Johnson is the first repeat winner at La Cantera since Leonard in 2000 and 2001. Leonard was a stroke back at 14 under through 16, but his chances at a record fourth Texas Open title ended when his 7-foot putt on 17 didn't reach the hole for par.
Next year, the tournament will move to a Greg Norman-designed TPC course.