St John’s Cathedral in Portsmouth is the Mother Church of the Catholic Diocese of Portsmouth. The Cathedral was built in 1882 to provide for the steadily increasing number of Catholics living in Portsmouth.

Prior to the late eighteenth century it had been illegal for English Catholics to have chapels within most towns and cities. The Second Catholic Relief Act of 1791 removed this prohibition. A group of Catholics in Portsmouth appealed to the Vicar Apostolic of the London District – which included the county of Hampshire and what was then the town of Portsmouth – for a priest whom they would support. The first appointed was the Rev. John Cahill who opened a chapel in a private house in the town. In 1794 he was succeeded by the Rev. Joseph Knapp who two years later opened a purpose built chapel located in Prince George Street, one of the backstreets of Portsea. With a seating capacity of 300 it had by the mid-nineteenth century become too small for the numbers attending Mass. Portsmouth being a garrison town had an ever-increasing number of Catholic soldiers and the number of local lay Catholics was also on the increase.