St Mary's is an exceptionally complete Romanesque structure, consisting of an unaisled chancel and nave separated by a central tower. It might originally have had an apsidal E end, but at some time in the 13thc the chancel was extended to the E, so the original termination is gone. The W facade is of 3 storeys: an elaborate doorway flanked by blind arches, then a storey lit by a large oculus, and in the gable a triplet with a blind arch above. The nave also has lateral doorways; that on the S side justly celebrated for its decoration. The vaulting of the chancel and especially the shafts supporting it, as well as the two tower arches, are also enriched with sculpture. Elaborately decorated windows are found on the W facade, and on the lateral walls of the nave, tower and chancel. The font is 12thc., as is an abandoned bowl in the churchyard.

The nave roof was lowered in the 17thc, and a crenellated parapet was added. This reduced the height of the W gable, cutting off the outer 3rd-storey windows below capital level. In 1823 the gable was rebuilt by Robert Bliss, and in 1844 the roof was raised to its original pitch by R C Hussey. Thus the blind window at the apex of the gable and the frieze below it are entirely 19thc work. The large oculus in the centre of the 2nd storey of the facade was inserted by J C Buckler in 1856-57, replacing a Perpendicular window. He was assisted in his restoration by an outline of an original oculus in the masonry. Apart from the work on the W front there was a general restoration of the church in 1844. In the 1970s the top storey of the tower was refaced, involving replaing much of the carved and moulded stone. In the 1980s a shelter coat of limestone spray was applied to the exterior sculpture, and this was renewed in 2017 by Sally Strachey Ltd.

In the 12thc. the manor of Iffley was held by Henry of Oxford, who exchanged it before 1156 with Geoffrey, son of Geoffrey Clinton the Chamberlain. By Geoffrey's grant it was held by the St. Remys, a Norman family. In 1158 Robert de St. Remy succeeded Richard. He probably built the present church and his daughter Juliana gave its advowson to Kenilworth Priory. The Clintons were founders of Kenilworth and the terms of Henry de Clinton's confirmation of Juliana's grants implies that he was overlord of the de Remys in Iffley.

This small but richly decorated church appears to have been built in one campaign some time between 1160 and 1180. The lavish use of chevron and beak-head decoration conforms to local sculptural tradition, but this cannot be said of the rosettes and figural motifs placed vertically on jambs and arches (S doorway) and on the shafts supporting the vaulting of the chancel. They seem to have no parallels in England. The use of Tournai shafts is also unprecedented in a small country church.

No detailed study of the sculpture has been published. Sherwood and Pevsner (1979) claimed that some of the decoration is unfinished, showing that it was carved in situ, not in the mason's yard as was usual. However, slight errors in fitting together the spiral decoration of the roll-moulded orders and the trimming of the two medallions on the label of the W doorway prove beyond doubt that the contrary was the case.

The S doorway of the nave was protected by a wooden porch (not original). Shown on engraving by J. Newton, dated 1785. Removed in 1807. (VCH, Oxford, V, 203).