Keith Fergus climbed into second place by himself at 9-under-par 133 after a second-round 66. Tom Jenkins and Wayne Levi, who shared second entering the round, are now tied for third place at minus-8 after 3-under 68s in the second round.
Gil Morgan, the first-round leader, birdied 16 and 17 before an eagle at 18 got him back to 1 under for his round. He shares fifth place with Mike Reid (65) and Ben Crenshaw (67) at 7-under-par 135.
Johnson had a stellar front nine. He opened his round with a 6-foot eagle putt at the par-5 third. The 50-year-old came right back with a birdie from 40 feet out at four to move to 7 under.
The Californian picked up another birdie at the sixth. Johnson converted his third birdie putt at the par-4 ninth, from 35 feet away, at Newport Beach Country Club.
Johnson continued to roll on the back nine with a 5-foot birdie putt at the 11th that moved him to 10 under. He kept going with a birdie at the 13th and another on the 15th.
He bogeyed the 16th for the second straight round to lose a stroke. Johnson, however, closed with a 3-foot birdie putt at the last to polish off a 63.
On Sunday, Johnson, a former beer truck driver, will go for his first Champions Tour title. This is the first time in his brief Champions Tour career that he has held a lead.
'Great day for me,' Johnson said. 'It was nice to have a good round for my mom on her birthday. I got off to a good start with an eagle and birdie at three and four. I was very fortunate to make numerous putts today. I'll be nervous on Sunday for sure.'
Fergus picked up birdies at the third and fifth to get to 6 under. After a bogey at the eighth, the 51-year-old birdied the ninth from 30 feet out to make the turn at minus-6.
Fergus, a three-time winner on the PGA Tour and a two-time winner on the Nationwide Tour, collected the fourth birdie of his round at the par-4 14th. He then sank a 6-foot eagle putt at the par-5 15th to jump to 9 under.
The Texan stumbled to another bogey at the 16th, but like Johnson, Fergus birdied the last to close out his round.
'I played solid today,' Fergus said. 'I made two bogeys, but made a few more putts than I did on Friday. Mark will definitely feel some pressure. It's up to us to put some more on him.'
John Bland, Dave Eichelberger, Mark McNulty and Lonnie Nielsen are in eighth place at 6-under-par 136.