From the Wikipedia page 
Westminster St James was a civil parish in the metropolitan area of London. It was also known as St James Picadilly, named after St James's Church, Piccadilly.
It was created in 1685 from part of the ancient parish of St Martin in the Fields, and was within the Liberty of Westminster. In 1855 the parish vestry became a local authority within the area of responsibility of the Metropolitan Board of Works. In 1889 the parish became part of the County of London and in 1900 it became part of the Metropolitan Borough of Westminster, with the parish vestry replaced by Westminster City Council. It was abolished as a civil parish in 1922.
It corresponded to the western portion of the contemporary district of Soho, the eastern part of Mayfair and the northern part of St James's.
To the north the boundary was along Oxford Street with St Marylebone, to the east it had a boundary with St Anne, Soho running along Berwick Street and Rupert Street. To the south there was a boundary with St Martin in the Fields, roughly along Pall Mall. In the west there was a somewhat more irregular boundary with St George Hanover Square, partly following Conduit Street, Old Bond Street and Dover Street.