Harrow was formed in 1934 as an urban district of Middlesex by the Middlesex Review Order 1934, as a merger of the Harrow on the Hill Urban District, Hendon Rural District, and Wealdstone Urban District.

The urban district gained the status of municipal borough on 4 May 1954. The 50th anniversary of the incorporation as a borough was celebrated in April 2004, which included a visit by Queen Elizabeth II.

In 1965 the municipal borough was abolished and its area transferred to Greater London under the London Government Act 1963 to form the London Borough of Harrow. It is uniquely the only London borough to replicate exactly the unchanged boundaries of a single former district. This was likely because its population was large enough. According to the 1961 census it had a population of 209,080 making it the largest district or borough in Middlesex.

Source: http://en.wikipedia.org/wiki/London_Borough_of_Harrow