When county councils were first created in 1889, it was decided that to let them have authority over large towns or cities would be impractical, and so any large incorporated place would have the right to be a county borough, and thus independent from the administrative county it would otherwise come under. Some cities and towns were already independent counties corporate, and most were to become county boroughs. Originally ten county boroughs were proposed; Bristol, Hull, Newcastle upon Tyne and Nottingham, which were already counties, and Birmingham, Bradford, Leeds, Liverpool, Manchester, and Sheffield, which were not. The Local Government Act 1888 as eventually passed required a population of over 50,000 except in the case of existing counties corporate. This resulted in 61 county boroughs in England and two in Wales (Cardiff and Swansea). Several exceptions were allowed, mainly for historic towns: Bath, Dudley and Oxford were all under the 50,000 limit in the 1901 census. Some of the smaller counties corporate—Berwick upon Tweed, Lichfield, Lincoln, Poole, Carmarthen and Haverfordwest—did not become county boroughs, although Canterbury, with a population under 25,000, did.
Various new county boroughs were constituted in the following decades as more boroughs reached the 50,000 minimum and then promoted Acts to constitute them county boroughs. The granting of county borough status was the subject of much disagreement between the large municipal boroughs and the county councils. The population limit provided county councils with a disincentive to allow mergers or boundary amendments to districts that would create authorities with large populations, as this would allow them to seek county borough status and remove the tax base from the administrative county.
County boroughs to be constituted in this era were a mixed bag, including some towns that would continue to expand such as Bournemouth and Southend-on-Sea. Other towns such as Burton upon Trent and Dewsbury were not to increase in population much past 50,000. 1913 saw the attempts of Luton and Cambridge to gain county borough status defeated in the House of Commons, despite the approval of the Local Government Board — the removal of Cambridge from Cambridgeshire would have reduced the income of Cambridgeshire County Council by over half.
Upon recommendation of a commission chaired by the Earl of Onslow, the population threshold was raised to 75,000 in 1926, by the Local Government (County Boroughs and Adjustments) Act 1926, which also made it much harder to expand boundaries. The threshold was raised to 100,000 by the Local Government Act 1958.
The viability of the county borough of Merthyr Tydfil came into question in the 1930s. Due to a decline in the heavy industries of the town, by 1932 more than half the male population was unemployed, resulting in very high municipal rates in order to make public assistance payments. At the same time the population of the borough was lower than when it had been created in 1908. A royal commission was appointed in May 1935 to "investigate whether the existing status of Merthyr Tydfil as a county borough should be continued, and if not, what other arrangements should be made". The commission reported the following November, and recommended that Merthyr should revert to the status of a non-county borough, and that public assistance should be taken over by central government. In the event county borough status was retained by the town, with the chairman of the Welsh Board of Health appointed as administrative adviser in 1936.