The Local Government Act 1919 had provided for the establishment of District Counties and County Councils by whole, groups of, or parts of municipalities and shires to perform nominated functions delegated by the Councils concerned, although the majority of those councils dealt with infrastructure or utility planning. In the Northern Beaches area, the idea of a County Council, specifically dealing with the control and development of electric utilities in the area shared by Manly Municipality and Warringah Shire first occurred in 1938 and again in 1943 but was postponed for consideration because of the Second World War. Manly and Warringah Councils considered the matter at a conference held on 17 March 1950, but Warringah Shire later withdrew its support for the proposal. However, later that same year, the Electricity Commission of New South Wales began investigations into the overal feasibility of the proposal. In its report of 14 March 1951 the Commission identified advantages of creating a county council comprising the two local government areas for the purpose of developing consistent electric utilities in the region. After further discussions in 1951, the councils of Manly and Warringah resolved to establish the Mackellar County Council consisting of three elected representatives from each Council to commence on 1 September 1951. The council adopted its name, following the 11 May 1949 proclamation of the local federal parliament Division of Mackellar, in honour of politician Sir Charles Mackellar and poet Dorothea Mackellar. The resolutions of these meetings were forwarded to the Minister for Local Government, Joseph Cahill, who consented to the establishment and Constitution of the Mackellar County Council which were proclaimed on 4 July 1951 and published in the NSW Government Gazette of 13 July 1951. The first delegates to the Mackellar County Council were elected by the Warringah Shire Council on 16 July 1951 and by the Manly Municipality on 17 July 1951. The inaugural meeting was held at the Manly Municipal Council Chamber on 15 August and commenced operations on 1 September 1951. The Council eventually moved to more permanent headquarters, with the administrative headquarters positioned between Manly Local Court and Manly Town Hall on Belgrave Street in Manly and the technical headquarters situated in offices next to the Warringah Shire Hall on Pittwater Road, Brookvale. On 14 September 1962 the Minister for Local Government and Highways, Pat Hills, opened the new headquarters building of the County Council, located on 48-52 Sydney Road, Manly. In May 1965, the employees of the County Council registered the "Mackeller County Council Employees Credit Union Limited". The Credit Union later operated as the Manly Warringah Credit Union. Then later became the Northern Beaches Credit Union and in 2016 merged with Community First Credit Union. In 1968 the County Council was involved in litigation involving its responsibilities under the law over the liability of adjoining properties to its developments. The Supreme Court of New South Wales ruled that under the ordinances of the Local Government Act 1919, the council had liability over damage to adjoining properties to any of its developments. The case of Anderson v. Mackellar County Council is now an oft-cited case of precedent in construction law in Australia. By the late 1970s, the NSW Government resolved to restructure of electricity distribution areas in New South Wales, which included the reduction of the number of electricity councils. This was implemented on 1 January 1980, with the Mackellar, St George, and Brisbane Water county councils being amalgamated with the Sydney County Council.
Chairmen
County Clerks
Common Seal
Soon after the Council's establishment, the councillors resolved to commission a 'Common Seal' to be the primary visual identifier for the council. The Council asked the eminent local historian and President of the Manly and Warringah District Historical Society, Percy Walter Gledhill FRAHS to draw up a heraldic design that represented the area over which the council had responsibilities. Gledhill, who had noted to the Sydney Morning Herald that "They wanted something historic, something to represent light and something from the so-called arms of Manly and Warringah. Those arms aren't registered either, you know", made the following description of his design to the council:
