St Mary’s is substantially a 12thc church with a W tower, nave, N aisle, chancel, and S porch. The tower buttresses and pinnacles and the clerestory of the nave are from the Perpendicular period. The S wall is of the 13thc. The E wall of the chancel was rebuilt during the 1861-62 renovation by Charles Kirk Jr. and restoration work on the nave and chancel roofs and W tower stonework was carried out in the 1990s. Romanesque elements consist of the W tower bell-openings, the S doorway, the tower arch into the nave, the N arcade of the nave, and the chancel arch.

Domesday Book records a priest and a church with one bovate of land attached to it at Syston in 1086, on land held by Guy of Raimbeaucourt. Around 1217, as part of its foundation endowment, the Augustinian Priory of Wroxton (Oxfordshire) was granted the advowson of St Mary, Syston, and in 1317 the canons actually acquired the church. Until around 1830, St Mary's served as the burial church for the Thorold family, landed gentry of Lincolnshire whose baronetcy at Marston goes back to 1642.

On the S doorway, the central portion of the lintel and lower part of the tympanum were removed at some point in the past and the doorway opening was converted into a pointed arch. The lintel and tympanum were restored to their present form during the 19thc. As a result, only the terminal figures and one-half of the penultimate figures to the L and the R in the lintel arcade are original; the rest of the lintel and the bottom centre portion of the tympanum are 19thc insertions. The original figures are much worn but it is clear that their hands were clasped in front of their chests; what is unclear is whether their hands were in the prayer-like position seen so emphatically in the 19thc restoration. As for the original arcade, the architectural details are quite simple, representing a column, plain capital, and arch system with no further design elaboration. The stark simplicity of the arcade with frontal figures is reminiscent of the design of the much earlier Anglo-Saxon Hedda Stone in Peterborough Cathedral. Taylor and Taylor read the damage to the far left side of the top of the lintel as evidence that the chip-carved arch above was inserted after the lintel was in place and conclude that the lintel is slightly earlier in date that the arch and tympanum. While this is possible, the evidence can also suggest a simple workshop error in laying out the relationship of design elements carved ex situ.

A vertical break on the L side of the tympanum suggests that it is made of two separate pieces of stone rather than being monolithic. Pevsner identifies the chip-carved motif on the arch of the S doorway as St Andrew’s cross; however, the inclusion of an additional “arm” extending from the centre makes this a five-point star motif.

The large, crisply dressed ashlars surrounding the earlier, rough, rubble masonry of the W tower suggest an extensive, later medieval rebuilding of the tower. Stocker and Everson put forward a rebuilding in the late 14th or 15thc and also propose that the current arrangement of the bell-opening features is probably not original but a result of this late medieval rebuild. Observations of the bell-openings for this analysis were made from the ground as the interior was inaccessible at the time; for further descriptive details on the bell-openings see Stocker and Everson.

The chancel arch appears to have had much intervention. Taylor and Taylor commented that the second order of the arch was 'more like the work of modern restorers than Normans' and the present author agrees. The detailed delineations of features on the human faces in the chancel arch capitals suggest a post-medieval reworking, as do the crisp, sharp edges of the imposts.

The tower arch is in excellent condition, though the first order of the arch appears to be comprised of replacement voussoirs as does the R side of the second order.

A good portion of the capitals of the N arcade in the nave have been renewed. The E respond is particularly crisp and appears to be completely renewed. On the capital of pier 1, the E face and a smaller section (0.16 m.) of the W face are good, with the rest renewed. The S half of the W respond capital is good. Both the E and W responds are off-centre of the arches which argues for a major intervention and explains the extensive renewal work.