McGrane kicked off his round at the Golf du Palais Royal course with an eagle, then finished in style with four birdies in the last five holes to earn a two-shot lead over Alejandro Cañizares.
"I played beautifully today and I'm delighted with it," said McGrane.
"I got off to a fast start and I managed to keep it going and finish it off nicely.
"It is testing out there at the moment, the greens are beautiful out here. If you play well you get plenty of opportunities and it's in pristine condition obviously."
England's Graeme Storm had looked set to join or surpass McGrane as he came to the last two holes seven under par, but a chip that rolled back to his feet on 17 cost him a double bogey before he dropped another shot at the last to finish in a tie for third alongside Kenneth Ferrie, Robert-Jan Derksen, Richie Ramsay, Edoardo Molinari, Phillip Price and Jorge Campillo.