McDowell shot a 5-under 66 and Simon Khan a 68 to take a one-stroke lead. Both bogeyed the short 17th hole at Loch Lomond, but McDowell birdied the last to draw even with Khan at 10-under 203.
Mickelson had five birdies and five bogeys for a 71 a day after his hole-in-one helped him make the cut.
Four players were at 204, including last weeks winner of the European Open, Ross Fisher (68). The others were Miguel-Angel Jimenez (68), Jeev Milka Singh (69) and Richard Green (70).
The third-ranked Adam Scott, who triple-bogeyed the first hole of Thursdays first round, had a 68 to be three shots behind for a share of seventh place.
Ernie Els shot 70 for 208 and a share of 19th, while Mickelson, who played with Els, was at 209 for a share of 24th.
The wind picked up Saturday and made low scoring difficult following the calm conditions Friday.
Mickelson started with birdies at Nos. 1 and 3, but misjudged his approach at No. 4 and finished in a bunker at the back of the green. He pitched from a tough lie eight feet past but missed the putt.
At the par-3 fifth, which he aced Friday, he finished 45 feet past the pin and three-putted.
It wasnt that bad of a round in tough conditions, but I just didnt quite get it going, Mickelson said. I made a couple birdies early but kind of stalled and didnt get the momentum going.
It was a round that could very easily been the round that I needed. I felt yesterday and today there was a low round waiting to come out and I hope it will be tomorrow.
McDowell, third in last weeks European Open, eagled the 14th to display the form that should guarantee a Ryder Cup debut in September.
Im trying not to look at the future, he said. Theres a lot of guys who want to be on that team and if I dont make it, Ill have chances in the future. Obviously, with the British Open looming next week, thats more important.
Its nice to be in the mix and in the last group again.
Khan had five birdies with two bogeys.
I was patient today, and this week I have managed my game well, said Khan, whose only European Tour victory came at the 2004 Wales Open.
Overnight leader Angel Cabrera shot a 78 and plunged to a share of 37th place.
Els said he struggled with his rhythm early after watching his South African rugby union team beat New Zealand on television.
I didnt get my normal warmup and I didnt have great rhythm at the start of the round, he said. Then, I got it back, but I didnt quite putt the way I putted yesterday and that was the difference.