McCullough shot a 6-under-par 66 to take the first-round lead in the 3M Championship in Minneapolis, Minn.
The 56-year-old leads Hale Irwin by two shots. Rex Caldwell, Doug Tewell and John Jacobs share third place at 3-under.
McCullough earned his maiden win in February at the Mexico Senior Classic. Just one month later he captured his sophomore triumph at the Emerald Coast Classic.
Friday, McCullough carded eight birdies and two bogeys at the TPC of the Twin Cities to take the pole-possession entering weekend play.
Today was exciting for me, to get to play with my friend and mentor Gil Morgan, said McCullough. Im not going to sit here and tell you I played good golf. I made five long putts. Im very lucky. Ill have to play a lot better the next two days.
McCullough made five putts of at least 40 feet in the first round.
Like his counterpart, Irwin is also in search of his third win of the season. This years Siebel Classic and Brunos Memorial champion offset back-to-back bogeys at the 8th and 9th holes with six birdies.
Full-field scores from the 3M Championship