The Church is a Grade II* listed building situated at the easternmost end of the village. St Leonard, to whom the Church is dedicated, lived in Gaul in the sixth century. He was the son of a noble family and gave up a career at court to become a monk. He is known as the Saint of all those possessed, either of evil spirits or of disease, and of all prisoners and captives - in fact of all who need to be unchained, physically or mentally. The first mention of the church, which is a rectory, occurs in 1223, when the Bishop of Lincoln collated Robert Haldein of Banbury to the church, which had been vacant one and a half years; the right of any patron was then reserved. In 1285 Aumary de St. Amand was patron and the advowson remained the property of the lord of the manor until the 19th century. At the sale of the manor to the Jervoises the advowson was retained by the Pigotts of Doddershall and has descended with that manor in the direct line to Vice-Admiral W. H. Pigott, the present patron. Samuel Clarke, annotator of the Bible, was rector of Grendon at the Restoration, but was ejected by the Act of Uniformity of 1662.

The church has a chancel measuring internally 33 ft. 6 in. by 16 ft., with modern north vestry, a nave 45 ft. by 22 ft. 6 in., and a west tower 10 ft. by 9 ft. 6 in.

The nave is probably in part of the 12th century, though having no details earlier than c. 1230; the chancel, which is set out on an axis inclining northwards from that of the nave, dates from early in the 14th century, and the tower is of c. 1460. The church was restored in 1866, when the east window was renewed, and the chancel reroofed. In 1902 various structural defects which had developed in the intervening period were remedied, the north-east angle of the nave being rebuilt. Commencing in 2017, following the theft of its lead, the roof was extensively refurbished – completion and the restoration of services achieved in 2019.

The chancel has a modern east window of five lights, but two original north windows and three on the south, all of two uncusped lights with plain pierced spandrels in the head; a moulded stringcourse runs below the sills, but has been cut away on the north wall, and there is a plain chamfered north doorway of two orders, originally external, but now opening to the modern vestry. The piscina is of more ornamental detail than the rest of the chancel, having a trefoiled head under a gabled label, inclosing a sunk trefoil, flanked by panelled and crocketed shafts, of which the eastern has been cut back to make room for a wall monument. In the recess over the basin is a moulded stone shelf. The chancel arch has two orders with plain continuous chamfers and broach-stops at the base and shows remains of mediaeval painted ornament on its soffit. All the seating is modern except for a 17th-century chair within the altar rails.

The nave is lighted by four windows, one at the north-east being like the chancel windows, but almost entirely renewed. The north-west window is of 15th-century date, of two cinquefoiled lights under a square head, and both windows on the south are of three cinquefoiled lights, the eastern of late 15th and the western of 16th-century date, with a straightsided pointed head. It is to be noted that the eastern half of the south wall is some 6 in. thicker than the western half, the latter being the later in date. There is no north doorway to the nave, but a break of masonry in the north wall may point to its former existence. The south doorway is the best piece of detail in the church, though much damaged; it is of two moulded orders, of c. 1230, with remains of foliated capitals and shafts to the outer order, which had in the arch large undercut foliated dog-tooth ornaments. The mouldings in both orders consist of rolls and deeply-cut hollows, those of the inner order continuing down the jambs. There are marks of the former existence of a south porch, and within the doorway is a 15th-century holy water stoup. Replaced at the south-east of the nave is a cinquefoiled piscina recess with a stone shelf, of late 14th-century date, which was moved in 1868 to the north-west corner of the nave. The hexagonal oak pulpit is in part of c. 1620, with carved panels set on a modern base. The nave roof is of late 15th-century date, of low pitch, with moulded tie-beams, under which are curved braces with traceried spandrels; it has undergone alteration and repair, and the western tie-beam and one purlin in the west bay of the roof are of different detail from the rest.

The tower is embattled, of three stages, with a stair-turret at the south-east, and has pointed twolight belfry windows with tracery; in the second stage are plain trefoiled lights on the north and south, and in the west wall of the ground stage is a window of three cinquefoiled lights above a fourcentred west doorway. The latter has a square label and trefoiled spandrels. The tower arch is of the full internal span of the tower, of three moulded orders, and two-centred.

The octagonal font is plain 15th-century work retooled and the nave seating is modern, but a few pieces of 15th-century bench-ends, formerly in the church, are now at the rectory. In the tower is an oak chest, probably of Elizabethan date. There are no monuments of importance, but in the chancel are 18th-century mural monuments to the Pigott family and to Richard Lord Saye and Sele (d. 1781), and another to his widow Christobella (d. 1789). There are also a certain number of late 17th-century headstones still remaining in the churchyard. Several incised sundials are to be seen on the south walls of the nave and chancel, and an incised cross on the south doorway.

There are three bells, the treble by Robert Atton, 1621, the second and third, of 1677 and 1664 respectively, from the Chandlers' foundry at Drayton Parslow.

The plate includes a covered communion cup of 1569, the cover being of 1570, and a plated paten and flagon. The registers begin in 1653.