Bruce Fleisher (68), Bob Gilder (68), Leonard Thompson (71), Rocky Thompson (71) and Allen Doyle (71) share third place at 5-under-par 139.
Mason, the overnight leader by two, flew out of the gate Saturday. He ran home a 15-foot birdie putt at the second and two-putted for another birdie at the third. Mason drained another 15-footer for birdie at the par-5 sixth hole to reach 11-under-par and build a four-shot lead over Irwin.
Trouble loomed late on the front nine for Mason. At the seventh, Mason knocked his approach over the green and hit a chip that left him with 50 feet for par. He three-putted from there for a double-bogey six and bogeyed the next hole to bring the field back into the tournament.
Irwin did nothing to take advantage of Mason's miscues. He birdied the second hole, but bogeyed the par-three eighth when he failed to knock his tee ball onto the putting surface.
Irwin did make a move on the back nine when he holed a 10-footer for birdie at 10. With the wind picking up Irwin could only muster one additional birdie, a nine-footer at the 15th.
'With today's conditions, the field played like they were trying not to lose the tournament than trying to win,' said Irwin, already a two-time winner on the elder circuit in 2002. 'I'm very happy with today. I didn't take myself out of it.'
Mason birdied the 15th hole to match Irwin and maintain a one-shot lead, but that would be short-lived. He made a mess of the 16th hole and made bogey, dropping him into the tie for first with Irwin.
'Other than No. 7 and No. 16, I was ecstatic with how I played,' said Mason, who won the NFL Classic this season as a Monday qualifier. 'It was tough out there. I have a chance to win the golf tournament.'
Full-field scores from the 3M Championship