Deane Pappas and Brad Quiri were tied for second place at 5-under 67, while Chez Reavie, Michael Sims, Jon Mills, Keith Nolan and Boyd Summerhays shared fourth at 4-under 68.
There were 20 players within five shots of McCarthy's lead at the end of the first round.
The leader bettered his season-low score by seven shots, scorching Fox Den Country Club with an eagle and six birdies.
'I tried to go with the 'Hit it, go find it, hit it again' mentality today,' said McCarthy, who needed just 22 putts. 'Sometimes I get too caught up in fairways and greens. I had a calmness about me today. I knew I was going to play well this week.'
Looking at his season to this point, there was no reason to believe the 24-year-old rookie would play this well.
Since making it all the way to the final round of last year's PGA TOUR Q- school, McCarthy hasn't made his first cut on the Nationwide Tour. He hadn't even shot a round in the 60s all season.
Thursday, he equaled the course record held by nine other players.
McCarthy credits the book Psycho Psybernetics for improving his attitude on the course.
'It talks about the power of positive thinking and if you imagine the end result, then you can get out of the way and let your body bring you towards that end result,' he said.
Pappas and Quiri both posted six birdies and one bogey for their 67s. Pappas is tied for second place despite shooting a 36 on his first nine holes.
'I played solid and made a few putts on the back side,' said Pappas. 'I'm just trying to keep my ball in play, get on the green and see what happens.'
Defending champion Hunter Haas shot a 71 and was tied for 34th place.