'It was just unbelievable, I just cannot believe it and what a finish,' said Johnson, who earned $247,500 for the win. 'I'll tell you, things have changed for me.'
Keith Fergus had closed within two strokes of Johnson, but was unable to get any closer as he polished off a round of 71 on Sunday. He shared second place at 9-under-par 204 with Wayne Levi. John Bland, first-round leader Gil Morgan and Don Pooley tied for fourth at minus-8.
Johnson wasn't at his best in the final round, but two late eagles proved to be the difference. He rolled in an 8-foot birdie putt at the first. However, he tripped to a double bogey at the next.
The 50-year-old bounced back with a birdie on No. 3 at Newport Beach Country Club. Johnson faltered to a bogey at the seventh, but his lead was still three on Fergus.
Johnson was unable to get up and down for par from in front of the green at 11. With that bogey, his lead dropped to two strokes.
With Fergus getting closer, Johnson took things into his own hands to make sure he wouldn't lose this tournament. He dropped a 5-iron 22 feet from the hole at the par-5 15th.
Johnson rolled in that eagle attempt to get back to minus-12. He stumbled to another bogey at the 17th. The Californian then lost his tee shot into the trees right of the fairway at the last.
Johnson calmly hacked a shot out of trouble and back into the fairway. From there, he pulled out a wedge for his third shot. The ball bounced past the cup, but spun back into the hole for an eagle giving Johnson his first ever Champions Tour title.
'The eagle at 15 put me in pretty good position. I hit a good 7-iron into 17, but just pulled it a little bit,' Johnson said. 'Then 18, wow, what a finish.'
Fergus had a chance to put pressure on Johnson, but struggled through the middle of his round. Fergus birdied No. 2, but tripped to a bogey at the next.
The 51-year-old Texan picked up another birdie at the sixth. However, Johnson faltered to three bogeys over his next four holes to fall off the pace. He got back into contention with birdies at the 12th and 16th, but could only par the final two holes.
Levi played the opening six holes at plus-1 after two birdies and three bogies. He slipped to plus-2 for his round when he bogeyed the par-4 11th.
The 53-year-old was able to get back into a tie for second as he birdied 13, 15 and 18 to close out his round.
Rodger Davis finished in seventh place at 7-under-par 206 after a final-round 68.
Jim Albus, 2000 champion Allen Doyle, Morris Hatalsky, Tom Jenkins, Bruce Lietzke, Mark McNulty and Dave Stockton were one stroke further back at minus- 6. Tom Purtzer, the 2004 champion, was among a group at 5-under-par 208.