McGirt and Jhonattan Vegas both shot an 8-under 63 at the TPC-Summerlin course, a layout that historically has yielded some of the lowest scores on the PGA Tour.
McGirt is in 137th place on the tour's money list and needs to move up to at least No. 125 to keep his playing privileges for next year. His best finish this season has been a tie for 22nd at the Phoenix Open.
Vegas was the surprise winner this year at the Bob Hope Classic and he followed that up by tying for third the next week at Torrey Pines in San Diego. He has faded since then but his victory ensures he will be on the circuit for the next two years.
Nathan Green and Charlie Wi shared third place at 64 and 11 more players were at 65 Thursday. Among that group was Nick Watney, one of two players in the field who competed last week at the Tour Championship.
Former British Open champion David Duval, who has made the cut just three times in his last 11 tournaments, was part of the crowd at 66.
McGirt has missed the cut 13 times in 28 events this year. He grabbed his share of the lead by making six birdies during an eight-hole stretch on the back nine.
This is the first of four tournaments that make up what the PGA Tour calls the Fall Finish. The events give players such as McGirt a final chance to keep their tour card for the following year.
Exploding whale video goes viral on Internet
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'