St. John's Co-Cathedral was commissioned in 1572 by Jean de la Cassière, Grand Master of the Order of St. John. It was initially named, in the Italian common language of the time, as "Chiesa Conventuale di San Giovanni Battista". The church was designed by the Maltese architect Girolamo Cassar, who was also responsible for the construction of many important buildings in Valletta. It is held that Cassar went to Rhodes to bring a plan of an already existing church that was by then converted to a Mosque, to use it as a model for the present Co-cathedral. However Cassar still took decisions over the final design and made modifications, and thus became the sole architect of the Co-cathedral. Once St. John's was completed in 1577, it became the new conventual church of the Order instead of St. Lawrence's Church in the Order's former headquarters Birgu. Construction of the oratory and sacristy began in 1598, during the magistracy of Martin Garzez, and they were completed by Grand Master Alof de Wignacourt in 1604.