The two-time defending champion was one shot back after the opening round and jumped out of the gate with two birdies over the first five holes under windy conditions at the Palmer Course at Turtle Bay Resort.
Irwin continued to plug along and drained a 30-footer for birdie at the par-4 11th to reach 6-under. Irwin was not as fortunate with the putter at the 15th where he failed to save par from six feet out.
The 57-year-old recovered with a birdie at the 16th after he hit his second shot within two feet of the hole. He gave the shot back at the 17th but closed with a five-foot birdie at the last.
'I don't feel like I've played my best golf yet,' said Irwin, who is looking for his fourth win at this event. 'If I play reasonably well, I like my chances.'
Mast picked up his first birdie of the day at the third but struggled with a bogey at the very next hole. At the par-4 14th, Mast hit a wedge to 30 feet and ran home the putt for birdie.
The 51-year-old rolled in a 20-footer for birdie at the 16th and two-putted for birdie at the par-5 last for a round of 67.
'You need to hit the ball solid in the wind,' said Mast, who is seeking his first win on the Senior Tour. 'When I made mistakes I was able to scramble. I'll try to shoot 67 tomorrow and see what happens.'
Smith carded a bogey-free 69 for his share of first while Hatalsky birdied the par-5 18th to complete the logjam.
'I felt like I played pretty well,' said Hatalsky, who won the Uniting Fore Care Classic in August. 'I missed a few short putts but overall I'm pleased with the way I played. Routine putts become very difficult in this wind.'
Gary McCord was one shot off the lead at 5-under-par 139. Bob Gilder, a four-time winner on tour this season, was one shot further back after a round of 66. Gilder finished alongside Isao Aoki, John Bland and Jim Albus at 4-under-par 140.
R.W. Eaks and Steve Stull, who shared the first-round lead with Albus, finished as part of a group at 3-under-par 141. The duo was joined by Dana Quigley, Sammy Rachels, Steve Veriato, Rodger Davis and Mark Pfeil in a tie for 10th.
U.S. Senior Open winner Don Pooley is four shots off the lead along with Jim Ahern at 2-under-par 142.
Andy North, Allen Doyle, Doug Johnson, John Schroeder, Roy Vucinich and Bruce Summerhays are tied at 1-under-par 143.
Full-field scores from Turtle Bay Championship