It "was a day of just hanging in there and letting things come. But I like the fact that I don't have to be perfect and I can still play here," Johnson said.
Reed shot a 63 in the morning session, just one year after missing the cut at TPC Deere Run and Glover finished a shot better at 9 under on Friday.
Australia's Matt Jones is a shot back at 11 under. Troy Matteson leads a pack at 10-under, while Steve Stricker is among those contending at 9 under.
Johnson has shot 18 consecutive rounds in the 60s at Deere Run largely by avoiding big mistakes and scrambling out of small ones and his ninth bogey-free round Friday was a tournament record.
"I'm just comfortable. I'm comfortable with every tee shot. I'm comfortable with every wind, and clearly I'm comfortable on the greens," Johnson said.
Catriona Matthew shot a 64 Friday to take a three-stroke lead after the second round of the Manulife Financial LPGA Classic in Waterloo, Ontario.
Matthew's 15-under 127 at the tournament's midway point blew away her previous career-best 36-hole mark on tour by five strokes. American Angela Stanford shot a 67 and was second.
John Deere Classic
Patrick Reed -12
Zach Johnson -12
Lucas Glover -12
Matt Jones -11
Daniel Summerhays -10