They were a stroke ahead of Steven Bowditch, Johnson Wagner and Brian Harman.
Three-time Deere winner Steve Stricker shot a 65 Friday for a 133 total to vault into contention at the par-71 TPC Deere Run, and was tied for sixth with Rory Sabbatini, Ryan Moore and Todd Hamilton.
Johnson, the Deere winner in 2012, shot a 4-under 67 in the afternoon despite two bogeys in his last four holes. That dropped him into a tie with McGirt.
McGirt, 35, is looking for his first PGA Tour victory.
He added a 66 to his opening 64, but like Johnson, he finished with a bogey. McGirt’s best finish this year is a tie for sixth in the Northern Trust Open at Riviera. Since then, he’s had one top 10 and has missed the cut in seven of 14 tournaments.
McGirt is winless on the PGA Tour in 111 starts, most of them in the last four years. His best finishes are a pair of ties for second in the Canadian Open, in 2012 and 2013.
Asked how he’d gotten to 12 under and the lead Friday, he said, “I couldn’t tell you what happened.”
U.S. Senior Open: Colin Montgomerie shot an even-par 71 on the second day and led Scott Dunlap by one stroke.
Montgomerie opened with a 65 on Thursday. Dunlap had a 69 in the first round and a 68 in the second to get to 5 under for the tournament.
Dunlap was at 2 under after starting on the back nine, and then went back to the front nine and birdied Nos. 2, 5 and 7 on his way to a 32.
Bernhard Langer, who leads the Champions Tour with three wins this season, moved into a tie for the lead with a 3-under 32 on the front nine, and then grabbed the advantage with a birdie on 14. He had bogeys on 15 and 16, both on close misses, and then bogeyed 18 to finish at 4 under.
Langer is tied with Gene Sauers for the tournament. Sauers shot 69 on both the first two days.
Marco Dawson shot a 66 on the opening day, good for second place, but shot a 76 on Friday and fell to even.
Scottish Open: Rory McIlroy surrendered the lead with his latest second-round meltdown, leaving three players tied for first place on a day when brutal winds played havoc with the field.
Kristoffer Broberg of Sweden, Argentina’s Ricardo Gonzalez and Marc Warren of Scotland were atop the leaderboard on 6 under, with just 17 of 150 players shooting below 70 on the par-71 Royal Aberdeen links course.
McIlroy shot a course-record 64 in the first round but followed it up with a 78, continuing a worrying trend this season that has seen the former world No. 1 fail to build on strong starts.
Justin Rose shrugged off the effects of hay fever to sign for a 68 and was alone in fourth place, a shot behind the trio of leaders.
Defending champion Phil Mickelson was among the afternoon starters who had to deal with a punishing breeze, especially on the back nine. A 73, which left him at 1 under, was something of an achievement but left him “mentally drained” just a week before the defense of his British Open title.
Cobb County’s Ollie Schniederjans, a Georgia Tech star and the world’s No. 1 amateur, shot 70 and joined Mickelson in the group at 1 under.
Englishmen Lee Westwood (73 for 3 over) and Ian Poulter (74 for 5 over) were the high-profile names to miss the cut.
Women’s British Open: American Mo Martin had a plan to play Royal Birkdale, and it’s worked better than she imagined.
The 5-foot-2 Martin took two putts from short of the green on the par-5 18th for her 10th birdie in two days. It gave her another 3-under 69 and a three-shot lead over Beatriz Recari of Spain and former U.S. Women’s Open champion So Yeon Ryu going into the weekend.
The weekend will not include Michelle Wie.
Coming off her first major in the U.S. Women’s Open at Pinehurst No. 2, the 24-year-old from Hawaii never got anything going and fell apart over the closing stretch for a 78 to miss the cut by three shots.
Only nine players remained under par and wind has yet to get fierce.