[3] Built out of Ancaster stone with a lead roof, St Denys' is furnished with a medieval rood screen and a communion rail, possibly by Sir Christopher Wren, and has a peal of eight bells, dating to 1796.

The church also houses several memorials, including two altar tombs commemorating members of the Carre family, Sleaford's lords of the manor in the 17th century.

[4] The Sleaford area has been inhabited since the late Iron Age; people settled around the ford where a prehistoric track running northwards from Bourne crossed the River Slea.

[13] The origins of New Sleaford are not clear, leading to a theory that it was planted by the Bishop of Lincoln in the 12th century as a means of increasing his income, hence the epithet "New".

Ramsey Abbey had been granted land in Sleaford and surrounding villages before the Norman Conquest of England; in Domesday its fee consisted of 1 carucate, 1 sokeman, 2 villeins and 27 acres of meadow.

The vicar could profit from tithes and oblations, and was given a house formerly occupied by one Roger the chaplain, but he had to pay £15 to the prebendary at the feasts of the Nativity and St John the Baptist.

A chantry chapel, dedicated to the Virgin Mary, was founded in 1271 by the merchants Thomas Blount and John de Bucham, who endowed it with lands around Old and New Sleaford, and several surrounding villages.

[22] The tower was probably accompanied by a nave of a similar date, which was rebuilt in the Decorated Gothic style in the mid to late 14th century; the transept followed twenty or thirty years later, according to Trollope.

[25] There is a widely held local tradition that St Denys' was used during the English Civil War (1642–51) as a barracks for parliamentary troops who destroyed the interior furnishings.

[26] The local historian Trollope stated that the soldiers looted the brass eagle lectern (last recorded in 1622),[27] broke the stained glass windows and the organ, and stole valuables.

[29] In 1772, Edward Evans, a ship's surgeon on HMS Egmont, donated £300 to replace the organ with one built by Samuel Green of London.

This coincided with a wider restoration project carried out at the cost of £3,500 by Kirk and Parry, which included the demolition of the galleries, the addition of a strainer arch and the relocation of the organ.

[29][34] The church was damaged by an electrical storm in 1884 and parts, including the stone broach spire—one of the oldest in England[35]—were rebuilt by Kirk and Parry in 1885–86.

[29] The old organ was sold in 1891 and St Hugh's Chapel and the choir vestry were dedicated to the memory of a local solicitor, Henry Snow, in 1906.

The earliest parts consist of the late 12th or early 13th-century tower and spire on the west side of the church, which have a combined height of 144 feet (44 m).

During the restoration, a 15th-century window was removed, placed in the churchyard and replaced by arcading and three circlets, deemed "somewhat absurd" by the architectural historian Sir Nikolaus Pevsner.

[39] Pevsner remarks that St Denys' is particularly notable for its tracery,[35] adding that "it is a prolonged delight to follow the mason's inventiveness along the building".

[2] The church's entry on the Statutory List of Buildings of Special Architectural or Historic Interest also notes its "particularly good mid [14th-century] tracery and ornament".

[19] The window above the north doorway is a good example; it contains elements shared by others of the Decorated style around the church, including reticulated ogee arches of varying complexity.

These have not survived,[44] but the current stained glass includes a "very Gothic" window by Hardman dating to c. 1853, three by Holland of Warwick from the late 1880s, one of Ward and Hughes (1885) and one by Morris & Co. from 1900.

[52][53] The church also houses a collection of fifteen antique, chained books in an oak reading desk; the oldest items date to the early 17th century and include tracts on divinity.

[41] The churchyard around St Denys' has been expanded several times: first in 1391, when the Bishop of Lincoln, John Bokingham, was granted a piece of land 150 by 8 feet (45.7 m × 2.4 m) to one side of the church.

These grounds were extended in 1862 by an acquisition of 3 acres and 39 poles (1.51 ha) of land to the west of the cemetery; they are now managed by Sleaford Town Council.

On the northern side of the chancel is an alabaster monument dedicated to George's eldest surviving son Robert Carre (d. 1590), his three wives and some of their children; he became lord of the manors of Old and New Sleaford.

Early examples are plaques to John Walpoole (d. 1591, monument dated 1631), the draper Richard Warsope (d. 1609, erected by Robert Camock), and Rev.