Television analyst Gary McCord, Morris Hatalsky and Champions Tour ironman Dana Quigley each carded rounds of 5-under 66. They share second place at 8-under-par 134. Allen Doyle and Bruce Fleisher are one stoke further back at minus-7.
McNulty opened with a birdie at the first and moved to 6 under with a birdie at the par-5 fifth. The 50-year-old faltered to a three-putt bogey at the par-4 seventh at Oak Hills Country Club.
The Zimbabwean bounced back with a tap-in birdie at the par-3 ninth. McNulty kept rolling around the turn. He eagled the par-5 10th to jump to minus-8.
McNulty, who won the Outback Steakhouse Pro-Am earlier this year in his tour debut, drained a birdie putt at the next before a par at the 12th. He came back with a 15-foot birdie putt at the 13th to move to 10 under.
McNulty made it two straight as he rolled in a 6-footer for birdie at the 14th. He capped a run of three straight birdies at the 15th, and closed his round with three straight pars to end at minus-12.
'I hit quite a few good quality iron shots,' said McNulty. 'My seven-hole stretch from seven to 14 did most of the damage. I was hoping to add one more down the stretch, but it wasn't meant to be. I'm delighted with my 63, but I can't count my chickens before they are hatched.'
McCord picked up birdies at the fifth and eighth to get to minus-5 overall. Around the turn, he dropped in a birdie putt at the 10th. The two-time winner on the Champions Tour sank back-to-back birdies from the 13th to move into a tie for second place.
Hatalsky stumbled to a bogey at the third, but ran off three birdies over a four-hole stretch from the fifth to get to 5 under. He posted six pars in a row from there. The 52-year-old birdied three of his last four holes to move to minus-8.
Quigley, making his 261st straight start on the Champions Tour, quickly got into red figures with a birdie at the first. He then converted birdies at the fourth and fifth, but tripped to a bogey at the seventh.
Around the turn, Quigley birdied Nos. 11 and 13 to get to seven-under. He drained an eagle at the 15th, but faltered to a bogey at the next to end at minus-8.
'It takes a lot of thinking on this course,' Quigley said. 'It is a lot different course than most we play. You've got to his it a precise distance. It is a whole different mindset.'
First-round co-leaders Tom Kite and Dave Stockton posted even-par rounds of 71. They share seventh place at 5-under-par 137 with Bobby Wadkins (67), Bob Gilder (68) and Darrell Kestner (67).
Ron Streck, Doug Tewell, Hale Irwin and defending champion Craig Stadler are one shot further back at minus-4.
James Mason and Tom Jenkins, who shared the first-round lead with Kite and Stockton, both stumbled to rounds of 2-over 73. They share 16th place at 3-under-par 139 with eight other players, including Gil Morgan who posted a tournament-record 9-under 62. That total matched the course record.