Montgomerie, the defending champion, played the closing seven holes in five under par for a round of 65, but Mitchell's 67 was good enough to give him a one- stroke lead over Montgomerie, David Gilford and Miguel Angel Jimenez.
Fearful for his career after key-hole surgery on both knees last November, the 36-year-old Mitchell has, instead, been given a new lease of life.
He was second to Carl Mason in the Bell's Scottish Open a fortnight ago, and after posting a halfway total of 132, 12 under par, he said: 'I don't feel so overawed when I find myself with the bigger names any more.'
That new-found confidence will stand him in good stead this weekend as he goes for only his second win in 20 years as a professional, since both the Masters champion, Jose Maria Olazabal, and the US Open champion, Ernie Els, fired 66s to move to eight under par.
Mitchell holed from 12 feet at both the 14th and 15th holes, came out of sand to five feet on the long 16th and made it four in a row by sinking a 15-foot putt at the 355- yard 17th.
Montgomerie, who lost a play-off for the US Open last month and was eighth in the Open championship at Turnberry last Sunday, said: 'I'm in contention again and that's what the first two rounds are for.'
Last season's European No 1, whose finishing burst began with a three-iron to four feet for an eagle at the 486-yard 12th, is third in this year's money list behind Olazabal and Bernhard Langer.
'My goals are to win the Order of Merit again and to bring my stroke average down, which I've done every year on tour,' he said. Yesterday's round brought that average below 70 for the first time and Montgomerie, asked if he kept records of such things, said: 'It's all logged in my mind - I'm 77 under par for the season and I've not been over par in any tournament in Europe.'
Gilford has still to have a bogey after 36 holes, while Jimenez sank a 20-foot eagle putt on the long 16th to make up for a double bogey five on the 144-yard 13th.
The Dutchman, Rolf Muntz, the British amateur champion four years ago, lies joint fifth with Howard Clark and Jeremy Robinson.
However, John McHenry, the joint first-round leader with Mitchell, Jimenez and Richard Boxall, crashed to an 80 and missed his 12th cut in 18 starts this season. The Cork golfer began and finished with a seven - one for a triple bogey, the other for a double.Reuse content