The oldest part of St Mary's is the 11th century nave, built with walls 3 feet thick made of puddingstone and flint. The brick tower was constructed in the early 16th century by the Knights Hospitalers, replacing an earlier wooden version. Like many other historic Essex churches, St Mary's was restored by the Victorian architect (and first mayor of Chelmsford), Frederick Chancellor. The restoration in 1869 included the addition of the tiled porch, the vestry and pews, while extensive remodelling was performed on the interior, as well as removal of the exterior plastering.