It was the 17th consecutive round in the 60s at TPC Deere Run for Johnson, who grew up roughly 100 miles away in Cedar Rapids, Iowa.
Matt Bettencourt made the field as a late alternate and shot 65. He’s tied for second with Daniel Summerhays and Brendon de Jonge.
Boo Weekley is among a pack of players at 66.
EUROPEAN TOUR: In Inverness, Scotland, Phil Mickelson quickly hit his stride at the Scottish Open, shooting 6-under-par 66 to sit two strokes off John Parry’s first-round lead.
Mickelson is seeking a first win in Europe in 20 years, while using the event as practice for next week’s British Open at Muirfield.
“It’s great for me to get off to a decent start because I have gotten off to poor starts the last couple of years here, and I have been fighting just to make the cut and get into reasonable contention,” said the four-time major winner.
The highest-ranked player in the field, No. 8 Mickelson chose some competitive play the week before the Open – unlike Tiger Woods, Rory McIlroy and Adam Scott.
LPGA TOUR: In Waterloo, Ontario, Angela Stanford and Catriona Matthew shot 8-under-par 63s to share the first-round lead at the Manulife Financial LPGA Classic, two strokes ahead of top-ranked Inbee Park.
Park is trying to become the first golfer to win four consecutive LPGA Tour events since Lorena Ochoa in 2008.
Park was joined by Irene Cho, Belen Mozo and Hee Young Park and Meena Lee at 65.
WEB.COM: In Sandy, Utah, Brian Prouty and B.J. Staten share the lead after the first round of the Utah Championship with 8-under-par 63s.
Tied for third place, at 64, are Kevin Foley, Adam Hadwin, Derek Fathauer and Matt Weibring.
Augusta’s Scott Parel opened with 70. Aiken’s Kevin Kisner had 71.