The village of Nether Heyford, with which it shares a primary school, church and other facilities, is about 0.5 miles (0.8 km) to the south.
The Council had previously served a court injunction against the developer to stop occupation of the property.
The enforcement notice will require the demolition of the new buildings and other works on the historic site.
With the intention of saving its historic character, the Council granted planning permission in 2005 to restore and change the use of several derelict buildings at Heyford Mill after they had suffered arson and vandalism attacks.
The site is in open countryside, part of the floodplain of the River Nene.