Two strokes off the pace when the delayed second round was completed earlier in the day, the 34-year-old journeyman took advantage of several favourable pin positions with superb all-round golf to fire a six-under-par 65.
McGirt birdied five of his first six holes, then picked up three more shots in the first four holes after the turn on the way to a 12-under total of 201 in the PGA Tour event at Riviera Country Club.
"Played really solid, hit a lot of good iron shots and good putts," McGirt told reporters after mixing eight birdies with two bogeys on a day of hazy sunshine and occasional cloud cover. "Just one of those days you could see the line."
George McNeill, a double winner on the U.S. circuit, carded a 66 after eagling the par-five first to finish at 10 under, level with fellow American Charlie Beljan (68).
Two more Americans, left-hander Brian Harman (68) and Monday qualifier Jason Allred (67), were a further stroke back in a tie for fourth.
(Reporting by Mark Lamport-Stokes; Editing by Gene Cherry)
