Between the end of World War 2 and 1964, two commissions on local government in England considered various possibilities for merging the counties, since their individual populations were not viable for the economic provision of services. There was opposition to combining all four counities into one. The final decision was to create two counties: Huntingdon and Peterborough, and Cambridgeshire and Isle of Ely.
The new county was formed on 1 April 1965 from the areas of the administrative counties of Huntingdonshire and Soke of Peterborough (with minor boundary changes) plus Thorney Rural District from the Isle of Ely. As well as becoming an administrative county, Huntingdon and Peterborough also became a county for other statutory purposes. Accordingly the Lord Lieutenant of Huntingdonshire became Lord Lieutenant of Huntingdon and Peterborough. Peterborough ceased to be part of the geographical county of Northamptonshire. A single commission of peace and court of quarter sessions were established for the new county.
At the next census in 1971, the county's population was 202,622.
The county was divided into thirteen local government districts: three municipal boroughs, three urban districts and seven rural districts. Each of these existed as subdivisions of the predecessor counties.
The county only had a nine-year existence. In 1974 the Local Government Act 1972 completely reformed administrative structures throughout England and Wales excluding Greater London. A system of metropolitan and non-metropolitan counties, each divided into districts was introduced. Huntingdon and Peterborough was merged with neighbouring Cambridgeshire and Isle of Ely to form the new enlarged non-metropolitan county of Cambridgeshire. Peterborough and Huntingdon became two of the county's six administrative districts and in 1984, following a resolution of the council, Huntingdon District was renamed Huntingdonshire.