The 40 year old carded eight birdies and two bogeys to lead by one shot from England's Richard Finch and South Korean Park Sang-hyun.
McGrane has just one previous European Tour title to his name - the 2008 Volvo China Open - but enjoyed a good round in Icheon with birdies going out on the 12th, 14th, 16th and 18th and at the second, third, sixth and eighth coming home taking him to six under.
“I made plenty of birdies and I was very satisfied with that and I putted well today and I had good pace on my putts which is important to me,” he said.
“If I get good pace, I can hole putts. And if my pace is incorrect, needless to say you don't get it. Obviously want to build on that and keep going.”
Finch produced a bogey-free round and concluded matters with a birdie four at the 18th to maintain the 33 year old's good form - the former Irish Open winner has had three top-11 finishes in his last seven events.
Park kept home interest going with six birdies, but finished with a 67 due to a bogey at the 15th.
Scotland's Richie Ramsay is alone in fourth after shooting four under par, while Ryder Cup stars Lee Westwood and Ian Poulter had mixed fortunes.
World Number One Westwood reached three under par - currently good enough for a share of fifth place - but bogeyed the seventh and shot a double-bogey six at the ninth, his last hole, to slip back into a share of 35th instead.
Poulter, the World Number 17, made an awful start with consecutive double bogeys at the tenth and 11th, before recovering somewhat with a birdie at the 14th and an eagle at the first.
He bogeyed the second to finish two over par.
On the course American Ryder Cup star Dustin Johnson reached two under through six.
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