The English county containing much of what is now London. In 1889 a new London County Council was created, taking out a large bite from Middlesex. In 1965 the Greater London council was created, absorbing even more of neighbouring counties, including all the rest of Middlesex, except the area around Potters Bar in the north, which went to Hertfordshire.
Thus Middlesex ceased to be an administrative entity. It was to some extent retained in postal names, though only referring to the outer countryside-cum-suburbia (such as Harrow, Hounslow, and Ealing), not the London suburbs that had been absorbed earlier.
Historically, Wessex, Essex, and Sussex were all long-lasting Saxon kingdoms. If there was ever a separate "Middle Saxon" kingdom of Middlesex, it was neither large nor long-lasting. London was originally ruled by Essex, then passed into the control of first Mercia then Wessex.
In 1889 the parts of Middlesex taken up by the new London county formed the boroughs of Bethnal Green, Chelsea, Finsbury, Fulham, Hackney, Hammersmith, Hampstead, Holborn, Islington, Kensington, Paddington, Poplar, St Marylebone, St Pancras, Shoreditch, Stepney, Stoke Newington, and Westminster. Parts of Surrey and Kent south of the River Thames were also included. Many of the boroughs were combined into larger ones in the 1965 change.