The 61-year-old Irwin, the 1998 and 2002 tournament champion who won the season-opening MasterCard Championship for his tour-record 45th victory, had an eagle, seven birdies and a bogey to match the course record on the Valencia Country Club layout.
'There were ideal scoring conditions out there,' Irwin said. 'The weather wasn't a factor, like last year. You could be more aggressive, there was no wind.'
Jim Thorpe, David Edwards and Isao Aoki opened with 66s, and Loren Roberts, Fuzzy Zoeller and John Jacobs shot 67s.
'I'm happy just shooting 66,' said Aoki, who had two eagles. 'I made a double bogey on 10 and told myself to just take it easy and be patient.'
Roberts broke Larry Nelson's tour record with his 33rd consecutive round at par or better. Nelson set the mark in 2000.
'I'm not a numbers guy, but I guess that is a pretty good thing to have on your resume,' Roberts said. 'Obviously, Larry Nelson held it for a long time.'
Defending champion Tom Kite and three-time winner Gil Morgan had 69s. Jay Haas, the Toshiba Classic winner last week in Newport Beach, also opened with a 69.
Irwin opened with a three-putt bogey on No. 10, then birdied the next three holes.
'I forgot about it by the time I got to the 11th tee,' Irwin said. 'I ripped a drive down the middle and then made three birdies in a row.'
Irwin added birdies on Nos. 1, 5, 6 and 8 and eagled No. 9.
'Most of my scoring came in clumps,' Irwin said. 'I had those three in the beginning and a late flurry at the end. In between, I was playing well, but not very productive.'
Peter Jacobsen, Scott Hoch and R.W. Eaks withdrew from the event. Hoch injured his left wrist and pulled out after nine holes. Jacobsen and Eaks withdrew midway through their rounds because of back problems. Jacobsen had laser back surgery two weeks ago to repair disk damage caused from a hip replacement last year.