The sale of Denver Mill is expected to complete shortly after an offer has been accepted.
Norfolk Historic Buildings Trust put the Grade II listed building on the market in October.
But now a sale has been agreed with a West Norfolk firm, which is hoping to open a tea room along with retail units.
James Parry, from the Trust, said: “There was a very good response to the mill going on the market, with interest from a range of potential purchasers, from local to international.
“We accepted an offer and we expect completion of the sale within the next few weeks.
“As part of the sale agreement the trust will be honouring its pledge to carry out the essential repairs on the mill tower and the work is expected to start in early spring.
“The new owner is local to West Norfolk, with a existing business, and is planning a tea-room plus a variety of retail units and events at the mill in due course.”
The mill, which was built in 1835 for John Porter on the site of an earlier post mill, was gifted to the county by Edith Staines in memory of her father and brother.
It had been owned by Norfolk County Council until 1995 when it was acquired by the trust, which has spent £1 million on restoration using a Lottery grant. It has also acquired other buildings on the site.
The four sails were removed in 2011 after a structural failure.
The trust is looking to spend around £120,000 of the money realised from the sale on repairing the mill tower.