A post mill is the most ancient design of windmill, and an open trestle post mill is the most exposed version of that design. The entire wooden body of the mill - the part that contains the millstones, the sack hoists, the gearing and the working space - sits balanced on a vertical oak post. That post in turn rests on a horizontal cross of two timbers called crosstrees, which are themselves carried on four brick or stone piers. The whole arrangement of post, crosstrees and piers sits in the open air, exposed to the weather, with no enclosing roundhouse around the trestle. To bring the sails into the wind, the miller pushes a long tail-pole projecting from the back of the body, which swings the entire structure - sails, machinery, miller, sacks of grain and all - around the central post. At Bourn the body measures fourteen feet long by ten feet wide. It stands thirty-one feet high at the ridge. The mill carries four sails - two Common sails and two Patent sails - on a wooden windshaft with a cast iron poll end. They drive two pairs of millstones arranged head and tail. The body was extended at the tail at some point to accommodate a bolter, the machine that sifts ground flour.

The documentary history of Bourn Windmill is a long sequence of ordinary local transactions. In 1653, Thomas Cook sold the mill to William Smythe, a blacksmith from Caxton, a village a few miles to the east. In 1779, John Butler, a farmer in Bourn, leased the mill; he bought it outright in 1799. Smythe died in 1832 and left the mill to his niece Mary Heywood of Huntingdon and her husband Elieze. The Heywoods sold the mill in 1836 to Joshua Hipwell of Toft for £550. Hipwell died in 1866 and the mill passed to his son William, then to Mr Papworth, then to Zaccheus Papworth by 1874, then to William Papworth, then to George Papworth. The mill worked by wind alone until 1924, when an oil engine was installed to supplement the sails on calm days. Commercial milling ceased entirely in 1927. By that point the mill had been grinding grain in some form for at least three hundred years, and probably much longer, and it was about to enter the most precarious phase of its life: that of an obsolete industrial structure with no commercial purpose, sitting on land whose owners had no particular reason to maintain it.

In 1931, two men decided Bourn Windmill should be saved. They were Alfred Bossom - a Conservative MP, architect and later a peer, who had made a fortune building skyscrapers in America - and Mansfield Forbes, an unconventional Cambridge English don famous for hosting riotous parties at his home Finella in Queen's Road. They bought the mill from a Mr Pentelow for £45 and engaged Hunt Bros., the millwrights of Soham, to undertake repairs. On 3 June 1932 they presented the restored mill to the Cambridge Preservation Society. The handover ceremony, by the standards of windmill openings, was unusually theatrical. Musicians turned up in Elizabethan costume. A man dressed as Don Quixote ceremonially tilted at the windmill. The deeds were formally accepted by A. B. Ramsay, the Master of Magdalene College and president of the society. Bottles of beer and cider were smashed against the brick piers by Mrs Bossom, Miss Batten, Miss Lloyd and Miss Spring, in the manner of christening a ship. The millwright Rex Wailes built a scale model of the mill with four Common sails, which was placed on display in the Children's Gallery of the Science Museum in London. The whole proceeding was the product of an interwar enthusiasm for English vernacular heritage that has now mostly gone, but that left in its wake a number of preserved windmills, watermills and farm buildings that would otherwise have fallen to ruin.

In October 2020 the worst possible news arrived for an open trestle post mill: rot had been discovered in the crosstrees - the two horizontal beams that carry the central post and therefore the entire weight of the mill body. The repairs done in the 1980s had infilled rotten cavities with cement, which had then trapped rainwater inside the timbers and accelerated the decay. Without urgent intervention the whole structure could collapse. The Cambridge Past, Present and Future charity, which had taken over the Cambridge Preservation Society's heritage portfolio, launched an emergency public appeal for the £50,000 they thought repairs would cost. The eventual bill was much higher. In November 2021 the mill was placed on Historic England's Heritage at Risk Register. Donations poured in. The National Lottery Heritage Fund awarded £148,456. Historic England gave a grant of £54,000. Public donations exceeded £20,000. Most unexpectedly of all, the architect Norman Foster - then in his late eighties, designer of the Hong Kong Bank, the Reichstag dome, the Millennium Bridge and a generation of modernist landmarks - gave his backing to the appeal. He had drawn Bourn Mill as a student at Manchester University in the 1950s, and had remembered the building ever since. The repairs were completed and the mill reopened to the public in April 2023, with its medieval post intact, its 17th-century deeds documented, and a small queue of visitors waiting to climb the wooden steps into a working space that has not entirely stopped doing what it was originally built to do.