'It's been a long time since I was in this position,' said Montgomerie, who has 27 European Tour victories to his name. 'It's nice to come back after three weeks off with the wrist injury and find some kind of form.'
Gregory Havret, who held the lead after 36 holes, carded a 3-under 69 to finish one shot back. He was joined by the English duo of Ian Poulter and David Gilford at 16-under-par 200.
Montgomerie was four shots off the lead to start the day at Simon's Golf Club and picked up an early birdie at the third. He added back-to-back birdies from the seventh to reach 12 under and knocked his tee shot to 16 feet for a birdie at the par-3 10th.
The Scot caught fire down the stretch with a birdie at the 15th and another at the 16th. Montgomerie then took advantage of the closing par-5s, reaching the green in two for a birdie at the 17th and hitting his second shot to the back fringe at the last.
Montgomerie took two putts for his fourth consecutive birdie to secure a share of the lead.
'Tomorrow is a very important day for me and I'm looking forward to it,' said Montgomerie, whose last win came at the 2002 Volvo Masters when he shared the title with European Ryder Cup Captain Bernhard Langer. 'I seem to have found a nice putting touch and yes, a win would give me a lot of confidence going to the PGA Championship. The game is based on confidence and I remember winning at Slaley Hall in 1997 and going to the U.S. Open at Congressional and shooting 65 to lead by three.'
Hansen, who plays this course regularly, tallied four birdies on the front nine to make the turn at minus-11. He converted a short birdie putt at the par-5 12th and like Montgomerie, Hansen surged to the top of the leaderboard on the closing holes.
'I felt confident,' said Hansen. 'It's my home course and I know every bump and bounce.'
The Dane birdied the 15th and hit his approach to five feet for a birdie at the par-4 16th. Hansen found a greenside bunker at the 17th but hit out to eight feet and drained the birdie putt to make it three in a row.
Hansen then reached the putting surface in two at the par-5 18th and two-putted for his fourth straight birdie and the 10th of a bogey-free round.
'I mentioned at the start of the weekend that it was important for a Dane to be up at the top of the leaderboard,' said Hansen, who earned his first title on the European Tour last year at the Irish Open. 'It's very important for the tournament and for Danish golf. This could be the springboard to something big.'
Poulter collected four birdies over his first seven holes and landed his approach inside seven feet for the first of back-to-back birdies at the par-4 11th.
The 27-year-old added a birdie at the 14th for a 65.
'It's going very nicely after a week off,' said Poulter. 'It's just a shame I couldn't pick up another couple of birdies at the end but 16-under par is definitely in there with a shout.'
Patrik Sjoland of Sweden eagled the par-5 18th to complete a round of 67. He finished alongside England's Brian Davis at 15-under-par 201.
Miguel Angel Jimenez and Kevin Na were two shots further back at 13-under-par 203.