An impressively large village church, St. Mary’s is made of Barnack stone and was probably an aisleless, cruciform church originally. A rebuilding in the mid-13^th c. saw the addition of the N and S aisles and in the later 13^th c. the building of the magnificent crossing tower with broach spire. The aisle arcades are from circa 1300 and the aisles themselves were heightened in the early 14^th c. In the 15^th c. the clerestory was added. There was restoration work on the window tracery and the parapet done by G. G. Scott in 1861-62 and T. G. Jackson remodeled the chancel in 1863. The roof decoration was added by Charles Nicholson in 1950. The Romanesque material here includes the central doorway of the W façade of the nave which dates from the late 12th c., two pieces of chevron fragments reset above the arch at the E end of the S aisle leading into the transept and two loose stone fragments.

In 1086 Domesday Book mentions Ketton as part of the land of King William I in the wapentake of Witchley in Northamptonshire and notes a priest in the village. Queen Maud granted land in Tixover in the parish of Ketton to the Bishop of Lincoln in 1104-06 and King Henry I confirmed this gift in 1123. The advowson of the church of Ketton may have been included in this gift as in 1146 Pope Eugenius III confirmed the church of Ketton to the canons of Lincoln Cathedral. The church was confirmed again to the dean and canons of Lincoln in 1163. Bishop Hugh de Wells granted a release of twenty days penance to anyone helping with the rebuilding of the church in 1232. On 7 October 1240 the church was rededicated by Bishop Grossesteste. As a prebendal rectory, a vicarage was ordained to it 1283 by Bishop Sutton and the church of Ketton was held by a canon of Lincoln Cathedral until the end of the 19^th c. at which time the vicarage became the gift of the bishop of Peterborough.

On the W doorway, the N capital of the second order, with its two angle volutes, may be a version of a Romanesque Corinthian capital which varied greatly in design composition – here the leaves have been placed in the upper zone of the capital. On both nook-shaft capitals of the second order the commonality of the drill hole in the center of the upper zone between each large leaf may be a tool mark to assist in the laying out of the design of the capital. In the W doorway arch, head #15, the human with a strap possibly across the mouth, is good medieval work, but the stone of the voussoir around the head is a new insert and the carved “A” on it also modern. However, the inscription between heads #17and #18 is authentic and may refer to a mason’s name rather than just a mark. The nook-shaft capitals of the 3^rd order, their adjacent capitals in the blind arches, and the imposts above these are all carved from a single piece of stone underscoring the unity of the original 12^th c. design here, even though this portal was disassembled and reset, likely in the 13^th c. renovations. The VCH and Pevsner both date this W doorway design to c. 1190.

Of the two large sections of reset chevron of the S aisle arch into transept, Pevsner notes that both are straight and probably parts of a jamb; however, the southern one clearly has a slight curvature which suggests they may have originally been part of a jamb and an arch of the earlier 12^th c. church.

Pevsner does mention the upper doorway into crossing tower bell chamber. Just above this doorway the late 12^th c. roof line can still be seen, the roof later being heightened during the 13^th century building campaigns.