Champions Run, where Jason Gore shot 59 in 2005, is known for low scores and the first round was no different. Of the 156 players in the field, 104 broke par and 121 finished the first round at par or better.
Matthew Every fired a 62 and is alone in second, one stroke clear of J.J. Killeen. There are six players tied for fourth at 7-under 64.
Mulroy played the back nine first at Champions Run and opened with a birdie on his first. He carded another birdie on 13 to get to minus-2.
After three straight pars, Mulroy made eagle on the par-5 17th and came right back with a birdie on No. 18 to turn in 5 under.
Mulroy, who won the South Georgia Classic back in April, birdied three and four, before making another eagle at the par-5 sixth.
'I thought if I made that putt I might have a chance to break 60,' Mulroy admitted.
Mulroy parred the next two, then birdied the ninth for the first-round lead.
'I was pretty relaxed all day,' Mulroy stated. 'I think you just get like that when you're playing well. Everything seems like it comes pretty easily.'
Every also started on the back nine and he got on the board with a birdie on the 13th. He drained back-to-back birdie chances from the 16th to turn in minus-3.
The 25-year-old poured in three straight birdies from the fourth to jump to minus-6. After a birdie on the par-3 eighth, Every made eagle on the short par-4 ninth to end one back at 9 under.
The group in fourth place includes Jin Park, David Schultz, Brian Smock, Jonas Blixt, Brent Delahoussaye and Tadd Fujikawa.