Johnsons score was his best in 17 career Senior PGA Tour rounds. This is just his sixth tour start.
Winless in his pro career since leaving college in 1972, Johnson has tried to Monday qualify 20 times this year.
Jim Thorpe is in second place after a 5-under 67. George Burns and Jose Maria Canizares are two back at 4-under.
Johnson made seven birdies Friday at Hayfields Country Club. His lone mistake came in the form of a bogey at the par-4 finishing hole.
In Johnsons last start, the Senior PGA Championship, the 50-year-old Wisconsin opened in 69 only to shoot a second-round 79 and fall from contention.
Full-field scores from the State Farm Senior Classic