Mickelson was in a group of four players in a tie for second after making six birdies in his last 11 holes for a 66, leaving the American talking up his improving links game ahead of next week's British Open.
Branden Grace and first-round leader John Parry were in that group with Mickelson after two more 66s, with unheralded Dane J.B. Hansen (69) making up the quartet.
Stenson, on 16 under par, has emerged from a prolonged slump and is showing the kind of consistent form that took the Swede to No. 4 in 2009.
"I wouldn't say I'm far off that," said Stenson, who was as low as No. 230 in the rankings 18 months ago.
"The way I played today, that was top-10 (in the world) standard. It's as good as I have hit the ball all season."
With the wind only arriving late in the day and the sun again beating down, Castle Stuart was largely defenceless for a third straight round but no player made the kind of hot round that could rip a bunched-up field apart.
Chris Doak led overnight on 12 under but the pressure on him to push on and become Scotland's first winner of its national open in 14 years seemed to weigh down on him. He could make only a 1-over 73.
As Doak tumbled down the leaderboard, birdies were coming thick and fast elsewhere. Gareth Maybin equalled the week's lowest round of 64, putting him in a three-way tie for sixth with Raphael Jacquelin (65) and Peter Uihlein (70), who struggled at times in the last group alongside Doak.
Accurate off the tee and creative as ever around the greens, Mickelson chose the ideal day to produce his full repertoire of skills.
American television was broadcasting a regular European Tour event live for the first time this weekend, with NBC fortunate enough to have two U.S. players -- Mickelson and Uihlein -- in contention.
Uihlein was patchy. Mickelson was anything but.
Refusing to get too frustrated after parring his way to No. 8, the four-time major winner delivered a barrage of four birdies in five holes playing into the wind. A delicate chip to within three feet set up another birdie at No. 16 and a 15-foot putt at the last lifted him briefly into the lead and produced one of the biggest cheers of the day.
"On the eighth hole, I just tried to slow things down a little bit ... I holed one putt and it ignited the round," said Mickelson, who was watched by his wife and three kids.
"The last 11 holes were a lot of fun for me to play."
Mickelson is seeking a first individual win in Europe in 20 years, and first ever in Britain. That would prove the ideal way to go into the British Open, one of the two majors he is yet to capture.
"I have kept the ball in play as well as I ever have (on links)," added Mickelson, who found 11 of 14 fairways on the undulating course in the Scottish Highlands.
Stenson has been under par in 24 of his 31 rounds this year but hasn't managed a tournament win since capturing the SA Open Championship in Johannesburg in November.
The biggest victory of his career came at the Players Championship at Sawgrass in 2009 and a year later he placed third at the British Open. It has been a tale of woe since then but he is on the way back, partly due to his improved mental approach.
"I have put down some long-term goals for my game," the 37-year-old Stenson said. "I feel like I'm playing golf with a little more ease and if it's not happening today, it will happen tomorrow or the day after.
"It's a little bit less stressful in that sense."
Matteo Manassero made the tournament's first hole-in-one -- acing No. 8 from 216 yards with a 3-iron hybrid -- but the BMW PGA Championship winner is nine shots off the pace after his 70.