A few Neolithic to Bronze Age flint artefacts have been found in Longwick-cum-Ilmer parish. These include flakes and scrapers found at Little Horsenden Farm and Rose Farm and cores, scrapers, a blade and some flint flakes near Longwick cross-roads. A watching brief at Ivy Cottage uncovered a pair of parallel ditches and two post-holes but these were undated. The Lower Icknield Way runs through this parish but recent arguments have been put forward to deny the prehistoric character of this supposed trackway.
Early Roman pottery and tile has been found in a fieldwalking survey south-east of Armour Farm, suggesting the site of a building. Iron Age to Roman pottery has also been found at Coldharbour Farm and Shrove Furlong Farm. Roman pottery sherds and a spindle-whorl were found at Mead Close in a fieldwalking survey.
In the medieval period Ilmer, Horsenden and Owlswick were separate manors to Longwick. South-east of Ilmer church there is a medieval moat which may mark the site of its manor house. From historical records we know that Ilmer had its own watermill from the eleventh to the fourteenth centuries, a manor house that was recorded in the thirteenth century and a deer park and windmill, recorded in the fourteenth. Owlswick is recorded as having its own chapel from the fourteenth century but it was destroyed in the Civil War. Horsenden also had its own watermill, according to the Domesday and later sources. The remains of an early watermill, possibly dating back to Domesday, were found in a watching brief at Longwick Mill when it was turned into a house. There seems to have been a medieval village around Stockwell Lane Farm as well, as there are house platforms and other earthworks around there. There are also three or four medieval fishponds at Anderson’s Farm.
The two main surviving medieval buildings in the parish are St Michael’s church in Longwick and St Peter’s in Ilmer. St Michael’s dates to the fourteenth century but has been altered in the eighteenth and nineteenth centuries. St Peter’s has a twelfth century nave and fourteenth century chancel and font. It also has three fifteenth to sixteenth century bells. Some of the secular buildings incorporate some late medieval fabric. These include Old Acres, which is a fifteenth to sixteenth century timber-framed house, and Bank Farm, which is mainly seventeenth century but does also incorporate the fifteenth century Askett Hall. Meadle Farm is a sixteenth to seventeenth century timber-framed farmhouse but has a fifteenth to sixteenth century timber-framed cruck-built barn in its grounds.
Horsenden House is now a nineteenth century manor house associated with a moat and fishpond, both of which may be post-medieval, probably seventeenth century, rather than medieval in date. There was probably an earlier house here, though, as there are records of Sir Edward Don, the lord of Horsenden manor in the early sixteenth century. Other listed buildings in the parish date from between the sixteenth and the nineteenth centuries. As we get further into the later centuries there is more industry. This is evidenced at Longwick with the nineteenth century railway track and bridge running between Princes Risborough and Haddenham and Thame Parkway. There are also the remains of the Watlington branch railway that was closed in 1957.