Its area was that part of the current City of Westminster which is north of Oxford Street, and east of Maida Vale and Edgware Road.

It included the areas Marylebone, Regent's Park, St John's Wood, and Lisson Grove, along with the western part of the district of Fitzrovia.

The name is derived from a chapel, dedicated to St Mary, and founded by Barking Abbey, the holders of the Manor of Tyburn.

The building is faced with Portland stone and is an example of Edwardian Graeco-Roman classicism, with a tower in the style of Christopher Wren and fluted columns.

[9][10] The metropolitan borough was divided into nine wards for elections: Bryanston Square, Cavendish, Church Street, Dorset Square & Regent's Park, Hamilton Terrace, Langham, Park Crescent, Portman and St John's Wood Terrace.