A former county (named after the Kingdom of the Middle Saxons), north of the River Thames which disappeared with the 1965 local government reforms.
It now exists mainly as part of postal addresses.
The part of the County of London that had been transferred from Middlesex was divided in 1900 into 18 metropolitan boroughs, which were merged in 1965 to form seven of the present-day inner London boroughs:
- Camden was formed from the metropolitan boroughs of Hampstead, Holborn and St Pancras
- Hackney was formed from the metropolitan boroughs of Hackney, Shoreditch and Stoke Newington
- Hammersmith (known as Hammersmith and Fulham from 1979) was formed from the metropolitan boroughs of Hammersmith and Fulham
- Islington was formed from the metropolitan boroughs of Finsbury and Islington
- Kensington and Chelsea was formed from the metropolitan boroughs of Chelsea and Kensington
- Tower Hamlets was formed from the metropolitan boroughs of Bethnal Green, Poplar and Stepney
- The City of Westminster was formed from the metropolitan boroughs of Paddington and St Marylebone and the City of Westminster.
Further information History of local government districts in Middlesex
Middlesex outside the metropolitan area remained largely rural until the middle of the 19th century and so the special boards of local government for various metropolitan areas were late in developing. Other than the Cities of London and Westminster, there were no ancient boroughs. The importance of the hundred courts declined, and such local administration as there was divided between "county business" conducted by the justices of the peace meeting in quarter sessions, and the local matters dealt with by parish vestries. As the suburbs of London spread into the area, unplanned development and outbreaks of cholera forced the creation of local boards and poor law unions to help govern most areas; in a few cases parishes appointed improvement commissioners. In rural areas, parishes began to be grouped for different administrative purposes. From 1875 these local bodies were designated as urban or rural sanitary districts.
Following the Local Government Act 1888, the remaining county came under the control of Middlesex County Council except for the parish of Monken Hadley, which became part of Hertfordshire.The area of responsibility of the Lord Lieutenant of Middlesex was reduced accordingly. Middlesex did not contain any county boroughs, so the county and administrative county (the area of county council control) were identical.
The Local Government Act 1894 divided the administrative county into four rural districts and thirty-one urban districts, based on existing sanitary districts. One urban district, South Hornsey, was an exclave of Middlesex within the County of London until 1900, when it was transferred to the latter county. The rural districts were Hendon, South Mimms, Staines and Uxbridge. Because of increasing urbanisation these had all been abolished by 1934. Urban districts had been created, merged, and many had gained the status of municipal borough by 1965.
- List of Lord Lieutenants of Middlesex
- Custos Rotulorum of Middlesex - List of Keepers of the Rolls
- List of High Sheriffs of Middlesex
- Middlesex (UK Parliament constituency) - Historical list of MPs for the Middlesex constituency
The Wikipedia page is 
- http://www.middlesex-heraldry.org.uk/publications/seaxe/SeaxeOS06-198501.pdf The Seaxe, Sixth Issue, January 1985, page 4