Horham is in the arable land of the East Anglian plain in central N Suffolk, 7 miles SE of Diss. The land falls away to the E of the village to the valley of a tributary of the Waveney, running form S to N, and on a plateau to the NW of the village is the site of a World War II US airfield. The church is in the centre of the village, alongside the B1117 Eye to Stradbroke road.

St Mary's comprises a nave with a S porch, chancel and an imposing W tower. The nave has 12thc. lateral doorways, the N blocked and the S under an 18thc. brick porch. The square-headed two-light windows all date fromc.1400. There is a NE rood stair. The nave roof has been raised. There is no chancel arch, and the chancel is the same width as the nave. Its walls rise as high as the nave walls before they were raised, but its roof is steeper so that both nave and chancel ridges are the same height. It was rebuilt in the 19thc., although it contains a 14thc. angle piscina in the SE window reveal. The windows are in the same style as those of the nave, except for a four-light geometrical E window. Nave and chancel are of flint; the nave mortar rendered and nave and porch whitewashed. At the W, the knapped flint tower appears to be early 16thc., and there were a series of bequests for the 'new' tower between 1489 and 1512, culminating in a bell bequest in 1514. The main patrons were the Jernegan family. It has a polygonal S stair, embattled parapet and flushwork on the plinth, buttresses, parapet and below the bell-openings. These bell openings are unusual, however. Those at the W, N and E are large two-light double-openings with reticulated tracery. On the S, the stair only leaves room for a single opening, but this has Perpendicular tracery. The main restoration of the church was by Augustus Frere in 1879-81, and he rebuilt the chancel and was responsible for the E window design. The tower was restored in 1984. Romanesque features recorded here are the two nave doorways.

The Domesday Survey records a holding of 1 carucate with woodland for 24 pigs and 2 acres of meadow and a church with 22 acres, held by Algar, a free man of Stigand, and two others, one a man of Bishop Aethelmaer and the other a man of Eadric of Laxfield before the Conquest, and by Judicael the Priest in 1086. Judicael the Priest also claimed 1 free man with 6 acres held by Herbert Blacun by order of Bishop Herfast. Under the holdings of Robert Malet were listed: 1 free man, Aelfric, who held 1 carucate with 4 acres of meadow and woodland for 30 pigs, and 2 free men who held 17 acres before the Conquest. These parcels were held by Robert de Glanville from Robert Malet in 1086. Also in 1086, Walter de Caen held 3 free men with 60 acres, 2 acres of meadow and woodland for 6 pigs, and Walter fitzGip held 2 free men with 1 carucate and 30 acres of ploughland, 3 acres of meadow and woodland for 28 pigs, both from Robert Malet. Under the holdings of St Edmundsbury abbey, 4 free men held 30 acres from St Edmund's in 1086. In the fief of the Bishop of Thetford, 7 free men held 42 acres and woodland for 16 pigs, and 2 free men held 19 acres. Finally, Roger de Candos held 1 free man from Hugh de Montfort, with 14 acres. Sir Hubert Jernegan of Horham was listed in the 13th-Century Red Book of the Exchequer as one of 22 knights holding lands in the Honour of Eye in the year 1166, and the Jernegans still held estates here when they contributed to the building of the tower of St Mary's in the early 16thc.

Benefice of Stradbroke with Horham, Athelington and Redlingfield.

Arch mouldings similar to both orders of the S doorway, along with scallop capitals, occur on the N doorway of St Martin's, Tuddenham, N of Ipswich.