The church has been rebuilt and re-rebuilt for nearly 800 years. The oldest visible work dates to the 13th century - the arch between tower and nave, the eastern corners of the nave, the western portion of the chancel, and a single original lancet window on the north side. In the first half of the 14th century, during the Decorated Gothic period that produced some of the most flamboyant tracery in English architecture, the building was substantially enlarged: aisles were added to the nave, a clerestory was opened above to flood the interior with light, and the chancel was extended eastward to accommodate a more elaborate liturgical arrangement. In the 15th century the masons came back and rebuilt the west tower in the Perpendicular style. In the 18th century someone added a brick porch with a graceful old 14th-century doorway reset in the new brickwork. The result is not a single building. It is a stratigraphy of devotion.

Pevsner notes a distinctive regional quirk in the Welwick nave windows. The masons of the East Riding during the 14th century commonly used square-headed window frames, but adapted the flowing Decorated tracery motifs to fit inside them - producing a hybrid form that Wesleyan and Victorian church builders elsewhere in England rarely attempted. The foremost example is at St Helen's, Skipwith, but Welwick has three of these square-headed aisle end windows and they sit beside more conventional pointed and segment-headed ones in a kind of medieval pattern book. Above the south aisle wall there is also an elaborate projection that is mysterious from outside until you go in and see what it serves. Country churches in places like Welwick are often pieces of architectural history that nobody photographs, full of details no guidebook bothers to describe.

St Mary's is Grade I listed - the highest category in the English listing system, reserved for buildings of exceptional interest. Roughly 2.5 percent of listed buildings in England carry this designation. The reasons are not obvious from the outside. There is no spectacular spire, no cathedral pretensions, no famous architect attached. What earned the listing is the unbroken quality of medieval workmanship, the dateable 1358 east window, the regional tracery types, and the intactness of the 14th-century scheme through eight centuries of country worship. The Virgin and Child carving in the porch niche has been weathered nearly featureless. The cobble and ashlar walls have been patched many times. Inside, the building is essentially what was finished in the 14th century, with later refinements. Welwick village around it is small. Most travellers on the A1033 to Hull never turn off to find it. But the masons who shaped these stones did the work to last, and the work has lasted.