Sydenham is located 3 miles SE of Thame in SE Oxfordshire. The church is a small building of flint and stone, with a wooden central tower with a broach-spire, dating mainly from the C13th and restored in the C19th. It boasts some Romanesque corbels in the chancel (Sherwood and Pevsner), but these are of wood and of doubtful date and so not included in this entry. However, there is a plain tub font, probably Romanesque.

The church is mentioned in a charter of 1185-6, but its early history is as obscure as that of the village itself (VCH). It was a chapel of the prebendal church at Thame in the C13th, perhaps before, and like it, also had the status of a peculiar. Although it separated from Thame in 1547, it came under its peculiar jurisdiction until the C19th. (VCH).

The church belongs to the benefice of Thame with Crowell and Sydenham.

The church was dedicated as the Assumption of the Blessed Virgin Mary (VCH, date apparently unknown), now known as St Mary.