The diocese was one of six founded by Henry VIII in 1539–40, with the churches of dissolved abbeys serving as their cathedrals.

[1] It was created from part of the Diocese of London, and comprised Westminster (which was raised to the dignity of a city), and the county of Middlesex, with the exception of Fulham.

[2] Henry established the new diocese by letters patent of 17 December 1540, which reconstituted the former monastery of St Peter, Westminster (today generally known simply as "Westminster Abbey") as a cathedral, with a bishop, dean and twelve canons.

[3] In 1552 an act of parliament of confirmed the status of Westminster as a "cathedral church and episcopal see to the bishop of London".

Westminster Abbey finally lost its cathedral status in 1556, during the reign of Queen Mary, when the chapter was abolished and the monastery restored.