St Mary's is an ashlar church with a nave with a S aisle and S clerestorey, a low W tower and a long chancel with a N organ chamber of 1878. The S arcade of the nave dates from c.1300, and the clerestorey from the same period or later. The S aisle was rebuilt in 1839. The N doorway is blocked and the south, of 1839, has a porch. The chancel arch has 12thc. responds and a 14thc. arch, and the chancel dates from the 14thc. too. The tower is Perpendicular with a battlemented top storey of 1673, to which period also belong the bell-openings and Wdoorway. The church contains an elaborately carved font, which could be 12thc. or 13thc., and this and the chancel arch are the only features considered here.

East Haddon was held by the Count of Mortain in 1086, and the manor had members at Brampton and Brington, and held the soke of Holdenby, which (according to RCHME) suggests that all these parishes might have been dependent on East Haddon. A priest was recorded, suggesting a church too. Sometime between 1155 and 1175, the church was granted by William de Dive to the Premonstratensian Abbey of Sulby (Northants), the grant confirmed by his son Hugh.

Benefice of Brington with Whilton and Norton and Church Brampton with Chapel Brampton and Harlestone and East Haddon and Holdenby.

A similar combination of arcading and foliage decoration is found on the fonts at Hinton-in-the Hedges and Magdalen College, Brackley, both dated to c.1190-1210 on account of their dogtooth ornament. Of these the Hinton font is closer to East Haddon, containing very similar foliage and arcaded registers in a different sequence, and must date from around the same time.