"It's unbelievable," Spieth, 23, told CBS Sports after becoming the first player since Tiger Woods to win nine times on the PGA Tour before the age of 24.
"Today I hit 17 greens, it was kind of a dream round for ball-striking when you are leading by a bunch and I finally got one (a birdie putt) to go on 17.
"Boring golf," double major champion Spieth said with a smile after totalling 33 putts on Sunday compared to 23 the previous day.
"I apologise but that was kind of the game plan. That's what was needed today and fortunately that's what we did to close it out.
"We could afford to make a couple of mistakes if I needed to ... fortunately we didn't and we just stayed ahead."
The Texan finished at 19-under 268, while fellow American Kelly Kraft, the 2011 U.S. amateur champion, fired a 67 for second place at 15-under, with compatriot American Dustin Johnson (68) a further stroke back.
Spieth, who won the Australian Open in November, has finished no worse than ninth in any of his past six starts worldwide.
He had produced a putting masterclass that yielded eight birdies at Pebble Beach on Saturday but his putter cooled down on Sunday as he recorded just two birdies at the same venue despite playing rock-solid golf from tee to green.
A two-putt birdie on the second hole was followed by a string of 14 consecutive pars that kept his closest challengers at bay before he picked up another at the par-three 17th, sinking a curling 30-footer before hoisting his putter skywards in celebration.
Asked by six-times major winner Nick Faldo how he had managed to putt so well on the bumpy poa annua greens at Pebble Beach during the third round, Spieth replied: "It's all speed work.
"I'm not thinking too much about the stroke, finding a point, kind of a mid-point in the putt, and dialling in the speed ahead of time ... training my hands for the speed of the greens and then from there just trying to hit it around that spot."
Australian world number one Jason Day, rebounding from a disappointing 75 in the third round, chipped in for eagle at the par-five 14th on the way to a 67 and a three-way tie for at fifth at 12-under.
(Reporting by Mark Lamport-Stokes in St. Augustine, Florida; Editing by Andrew Both)
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