In 1888, he designed St John's Cathedral in Napier. This brick construction was demolished in the disastrous 1931 earthquake that destroyed much of Napier. Between 1886 and 1897, Mountfort worked on one of his largest churches, the wooden St Mary's, the cathedral church of Auckland. Covering , St Mary's is the largest and last timber church built by Mountfort, and the largest wooden Gothic church in the world. At its completion, it was said that "in point of design, completeness and beauty [it] reaches a high level mark not yet approached in the diocese". The emphasis placed on the sweeping roof by the great aisle windows struck a balance to the great area the church enclosed. In 1982 the entire church, complete with its stained glass windows, was transported to a new site, across the road from its former position where a new cathedral was to be built. St Mary's church was consecrated in 1898, one of Mountfort's final grand works.