McCarthy, a 24-year-old rookie, was at 10-under 134 for a one-shot lead over Nationwide Tour veteran Deane Pappas.
He made his first cut in three events this season after his course record- tying 64 on Thursday marked the first time he had broken 70.
'To know that I've cashed a check, no matter what happens on the weekend, is huge,' McCarthy said. 'This will help me have a much better chance of playing as the year goes on.'
He had problems on the front nine Friday, making two bogeys in a three-hole stretch from the third and eventually making the turn in 38. But he shot a 32 on the back nine to hang onto his lead.
'I just didn't hit it as straight as I did yesterday, so that was a problem,' McCarthy said. 'I just stayed within myself and played solid through the day.'
Pappas had a 4-under 68 in the second round and was alone in second place at 9-under 135. He hasn't been in this position in two years.
'My plan at the start of the week was the shoot 20-under par. So I'm close to half way there,' said Pappas. 'I'm just trying to hit my spots.'
Brad Fritsch fired a seven-under 65 and was alone in third place at 8-under 136. Roland Thatcher (66), Brad Quiri (70) and Jon Mills (69) were another stroke further back at 138.
The cut line fell at 1-under 143 with 67 players moving on to the weekend.