Irwin owns the most wins in Champions Tour history with 45, but his best finish at this event is a tie for seventh in 2004.
The 61-year-old Irwin has won five of the last six events he has led after 36 holes, including the MasterCard Championship at Hualalai earlier this year.
Irwin led by two entering play on Saturday and was never tied during the round. He did stretch his lead to as many as three strokes early on.
Andy Bean (66) and Bobby Wadkins (67) each flew up the leaderboard with bogey- free back nines. Bean, the runner-up at the Outback Steakhouse Pro-Am, and Wadkins, who won the ACE Group Classic, are tied for second at minus-8.
Tim Simpson matched Bean's 66 for the low round of the day. The 66 moved Simpson into a share of fourth at 7-under-par 137. He stands alongside three-time champion Gil Morgan, Morris Hatalsky, Loren Roberts and Jim Thorpe.
Roberts carded a 2-under 70 Saturday. That stretched his tour record of consecutive rounds of par or better to 34. He surpassed Larry Nelson's record of 32 on Friday.
Irwin, who matched the course record at Valencia Country Club on Friday, dropped in a 12-foot birdie putt on three to extend his lead to three.
After Hatalsky closed within one, Irwin stuck his approach within 2 feet at the par-4 eighth. He tapped that in for birdie to move to 10 under.
Irwin, who broke a 15-month winless drought with his victory at Hualalai, drained a 25-foot birdie putt at 11 to again move two clear of Hatalsky.
'I made a nice birdie on eight. Later, I made a poor drive on 12 and after that point, I just didn't play very well,' Irwin explained.
Hatalsky dropped shots at 13 and 16 to end two back. Irwin also bogeyed No. 13 after he missed the green and could not get up and down for par.
Irwin missed the green again on 17 and found a similar result with a bogey that dropped him to minus-9. He two-putted for par at the last to remain there.
'I let a lot of opportunities slip away,' said Irwin. 'I lost focus today a little bit. The pace of played seemed slower today and I didn't adjust to it and that hurt my rhythm.'
On Sunday, Irwin will look to become the first player over the age of 60 to win twice in a single season on the Champions Tour.
Wadkins was 1 under par for his round through seven holes with three birdies and two bogeys. Birdies on nine and 11 moved him to minus-6. The 55-year-old collected back-to-back birdies from the 14th. He parred the final three holes to end at minus-8.
'I made a couple nice putts on 16 and 17 for pars. That is the whole key,' Wadkins said. 'You make some putts to keep your round going.'
Bean posted six birdies in a bogey-free round. David Eger was the only other player to card a bogey-free round Saturday.
'I played steady today and had a lot of birdie opportunities,' Bean commented. 'I drove it better today than yesterday. I played good (Friday), but could have shot better.'
Eger shot 5-under 67 to move into a tie for ninth at 6-under-par 138. He was joined there by former Senior PGA champion John Jacobs, Wayne Levi, 2003 winner Tom Purtzer, Bruce Summerhays and Fuzzy Zoeller.