There are records that there was a church at this location of current St Mary's Church before the Norman conquest of Wales as it is mentioned in Domesday Book and by a charter from King John of England. However, there is nothing surviving in the current church building from that period. This is reportedly because of Augustinian canons from Llanthony Secunda visited and built the church on top of the older church that was dedicated to St Bride dating from around AD 900 at the behest of Walter of Gloucester after the construction of Caldicot Castle. The earliest part of the church is the base of the tower, which comes from the 14th century, along with the nave and chancel, which was part of an enlargement programme at the time. The tower was not fully completed until the 16th century.
In the 1850s, the 15th-century north aisle was rebuilt by Henry Woodyer, as well as most of the stained glass windows being replaced. In 1905, the chancel was refitted and many of the Victorian additions were changed.
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.