There was a church on the site of St Mary's since circa 1328, erected by Henry de Gower, Bishop of Saint David's. One Sunday morning, in 1739, the roof of the nave collapsed into the church while the congregation was waiting to enter the building. The whole structure was re-built apart from the tower. 1822 saw the church being lit by gas for the first time with thirty six lamps. The church underwent complete renovation between 1879 and 1882 by Vicar Dr Morgan. In 1896, the church was flattened and rebuilt again under the designs of Arthur Blomfield by Dean Allan Smith, though some parts of the old church survived the re-development. In February 1941 the church was extensively damaged by Bombing during the Blitz. It was not rebuilt until the 1950s.
From the 1890s the Swansea Devil stood on a set of buildings facing the west side of the church, constructed by a disgruntled rival of Blomfield's, angry at the commissioning of Blomfield's designs over his own.
References:Visby Cathedral (also known as St. Mary’s Church) is the only survived medieval church in Visby. It was originally built for German merchants and inaugurated in 1225. Around the year 1350 the church was enlarged and converted into a basilica. The two-storey magazine was also added then above the nave as a warehouse for merchants.
Following the Reformation, the church was transformed into a parish church for the town of Visby. All other churches were abandoned. Shortly after the Reformation, in 1572, Gotland was made into its own Diocese, and the church designated its cathedral.
There is not much left of the original interior. The font is made of local red marble in the 13th century. The pulpit was made in Lübeck in 1684. There are 400 graves under the church floor.