Johnson set the pace on Thursday in fairly benign conditions, although he was one of only a few players to make it round 18 holes at the Old Course without a dropped shot.
Playing alongside Jordan Spieth, Johnson sprinted out to four under after five holes, making eagle at the fifth, and he managed three more birdies in a fine round of 65.
With the weather expected to take a turn for the worse on Friday with heavy rain and wind predicted, 2011 runner-up Johnson said putting down a low number had been his priority.
“Everybody knows the weather Friday and Saturday is going to be very difficult, so today I thought was very important to get off to a good start and try to make as many birdies as you can,” said Johnson, who three-putted the 72nd hole to hand the US Open title to Spieth last month.
“The next couple of days it’s going to be very difficult … golf is going to be interesting tomorrow, but I enjoy the challenge. I like it.
“You’ve got to be very creative. You’ve got to use your imagination a lot when you’re out there when the wind is blowing that hard.”
Johnson’s biggest escape came at the Road Hole – the 17th – where he had to drain a long putt for par, and he admitted scoring had started to get tougher during the back nine.
Only two of Johnson’s birdies came after the ninth, while Spieth was one-over-par for his last seven holes despite picking up a shot at the last.
“I just got off to a good start, played well on the front nine, and then coming on the way back in, it played pretty difficult,” Johnson added.