The building was designed by the late Mr. M F Cavanagh, architect, in the English style of Perpendicular Gothic. The builders chosen for the work were A T Brine and Sons. The main building stone was limestone. A Sydney firm, Melocco Brothers made the sanctuary mosaics based on designs taken from the ancient Irish Book of Kells. All stained glass windows were designed and executed by the firm of John Hardman of Birmingham, England.
In 1973 Archbishop Sir Launcelot Goody, 3rd Archbishop of Perth decided to bring the sanctuary area into line with the liturgical reforms of the Second Vatican Council. The central altar and lectern were added to the sanctuary. The redesigned sanctuary was consecrated on August 12, 1973.
On 27th August 2006, the Cathedral was closed by Archbishop Barry James Hickey, 5th Archbishop of Perth, for construction to begin on the Conservation and Completion of the Cathedral. During this time of construction St Joachim’s Parish Church, Victoria Park became the Pro-Cathedral. The Cathedral was reopened on 8th December 2009, the Feast of the Immaculate Conception of the Blessed Virgin Mary which is the Patronal Feast of the Cathedral.