McGrane had eight birdies to counter a pair of bogeys at the Blackstone Golf Club in Icheon. Felipe Aguilar, Richard Finch and Park Sang-hyun are tied for second at 5-under 67. Park had six birdies and a bogey but Aguilar and Finch were both bogey free Thursday.
Richie Ramsay and Chinnarat Phadungsil are another stroke back at 4-under.
McGrane played the back nine first Thursday and was 3-under after seven holes before slipping with a bogey at 17. He came back with a birdie at 18 and made the turn at 3-under.
He got to 5-under with birdies at Nos. 2 and 3. He had a bogey at the 536-yard, par 5 fifth hole but again quickly answered with a birdie at 6. A birdie at No. 8 got him to 6-under.
"I made plenty of birdies and I putted nicely," McGrane said. "I holed everything inside 15 feet."