In all, nine players were within six shots of the lead.
"The conditions made it tough - the rain and then the wind", Mickelson said. "That was weird", he said. I always think about the Open like this - that you want to get all sorts of conditions. The French flag that flies above the grandstand at the 18th green, along with the banners of every other nation represented in the 156-player field, was lowered to half-staff. That 59 in Hawaii for the Grand Slam of Golf back in like '05, I've done that, and I'll have opportunities under the right conditions.
Mickelson joins a long list of greats left shaking their heads and fists at the golfing gods for their wicked vigilance. 15-17, to shoot 31 on the inward half, the left-hander gained almost six strokes on the field.
"It's one of these golf courses that it allows you to get off to a quick start and allows you to get nearly overconfident and cocky".
World number three Jordan Spieth (level) and England's Justin Rose tee off at 14:04, while world number one Jason Day (+2), Masters champion Danny Willett (level) and world number seven Rickie Fowler (-2) tee off at 14:37.
Scandinavians Soren Kjeldsen and Henrik Stenson reigning champion Zach Johnson, from the United States, also enjoyed the benign early weather to move to six under par.
Despite dropping shots at 15 and 18, the reigning champion was handily placed in fifth - five behind leader Phil Mickelson - and seemingly unlikely to slip down the leaderboard as numerous afternoon starters struggled in strengthening winds.
At 46, Mickelson would be the oldest Open champion since 1867, though he was quick to dismiss the relevance of the statistic.
Mickelson doesn't need to shoot a 63 every round to win, but if he can maintain the low scores, he will be in good shape.
Rory McIlroy is 4 under through 12 holes on his first round at the British Open since lifting the claret jug in 2014.
Stenson, former world number one Lee Westwood and four-times major victor Ernie Els were among several other players wearing black ribbons on their headgear. A warning for Mickelson: McIlroy shot 80 on the Friday.
"It became obviously more hard, but I enjoy that challenge". Put myself in with a chance come Sunday. "A good number might be over par".
"I was taught growing up that to hit the ball low, you scoot the ball back in your stance, which de-lofts the club", Mickelson said.
If only that had lipped in instead of out, he would have shot the first ever 62 at a Major.
"There's been two or three majors where I've been right up there, " he said.
"I've just got to go out and play tomorrow and see what happens after that and evaluate after the third round and go on from there. It was, wow, that stinks".
On a day of glorious sunshine on Scotland's west coast, Mickelson's round was flawless, with no dropped shots and eight birdies.
Most encouraging of all: The lead threesome took just under two hours to complete the front nine.
Luke Donald, playing alongside Montgomerie in the first group, was at 2 under. And he took the crowd with him.
Former Auburn player Michael Johnson shot a 65 to reach 10 under in his pro debut.
"When I've won the Scottish and British there was a warmth that I felt being here, and it's been special".