The announcement was made at an Olympic Summer Ball fundraiser - run by the town's Armchair Club organisation - at Mansfield Civic Centre, where dentist Dr Hancock was among more than 100 guests in attendance.
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Cheers rang around the room as Mansfield businessman and compere for the evening, Bill Taylor, made the announcement.
Dr Hancock - who took over as chairman of non league football club Glapwell last year with ambitions to lead the club into the Unibond League - told Chad.co.uk late on Saturday night that the deal involved the purchase of the club and Field Mill.
He will join the club immediately as an executive director ahead of an official handover - but would not disclose details of the figures involved.
And he reassured sceptical fans - who have been left disappointed in the past year following the collapse of the James Derry-led buyout - that the deal WOULD go through.
He said 'heads of terms' had been agreed and stressed he would not have been prepared to have gone public in such a manner if he felt a deal was not on the cards.
Dr Hancock, who lives in the Nottingham area, said he had been involved in a series of talks with Mr Haslam in recent weeks - but only received news that the under-fire Stags owner was happy to sell to him on Saturday evening.
"I was literally pulling into the car park here (at the Civic Centre) when I got the call from Keith," he told Chad.co.uk.
"I'm going to go in straight away because the place needs tidying up."
Dr Hancock first shot to prominence earlier this year when he rang into a BBC Radio Nottingham football phone in to reveal he had funds in place financially back Notts County.
And he was part of a group which was involved in serious negotiations, but says this waned when the supporters trust made it clear it wanted to own County.
Then it emerged that he was pursuing a possible buyout offer at Field Mill.
He was introduced to James Derry as a potential partner in what would prove to be a failed consortium, which three times went close to securing ownership of Mansfield Town FC earlier this year.
In February - when Chad first became aware of Dr Hancock's involvement - he said: "I am an honourable man. I would not do anything to adversely affect a deal happening at Mansfield. I just want to see the club progress.
"When I could not go any further with Notts, the logical thing to do seemed to be to look closer to where I was born and to Mansfield, which also needs help."
The Hancock deal would bring to an end a controversial 15-year stint at the helm for Haslam, who has been the subject of fans anger for several seasons.
It would also extinguish the hopes of eccentric entrepreneur John Batchelor's hopes of buying the Stags, while there has also been speculation of a bid being made a group of Mansfield businessman.
For more on this story go to chad.co.uk.