The change from a rural district to an urban district occurred in 1934; with the new district retaining the same boundaries. The urban district was transferred from Middlesex to Hertfordshire in 1965.
South Mimms had been the only parish from Middlesex in the Barnet rural sanitary district, the rest of which had been in Hertfordshire and had gone on to form the Barnet Rural District of that county.
The South Mimms parish and rural district was affected by minor boundary changes in 1894, 1896, 1924 and 1926 In 1934 the rural district was abolished.
The Royal Commission on Local Government in Greater London considered the district for inclusion in Greater London. However in 1965, when Middlesex was abolished and its former area was transferred to Greater London under the London Government Act 1963, the Potters Bar Urban District was instead transferred to Hertfordshire.
In 1974, under the Local Government Act 1972, the urban district was abolished and its former area was combined with that of Elstree Rural District, Bushey Urban District and part of Watford Rural District to form the present-day borough of Hertsmere.