The 1865 Prisons Act set strict rules governing the design of prisons, cells, hard labour, solitary confinement, diet and administration. As Ruthin Gaol did not conform to the new standards the Ruthin Gaol Committee was set up in January 1866 and it was decided to build a new four-storey wing. By 15 March plans of the alterations were ready and it was:

"...ordered that such part of the walls of the present gaol as require to be altered in order to carry out the intended improvements... be pulled down at once by the prisoners under the direction of the County Surveyor."

Built in the style of Pentonville (erected 1840-42) the new prison could hold up to a hundred prisoners and cost £12,000.

Plans of Ruthin Prison in 1866 (Denbighshire Record Office).

On 1 April 1878 the County Gaol Ruthin became H.M. Prison Ruthin serving Denbighshire, Flintshire and Merioneth. Further alterations were made to improve conditions for female prisoners and the prison acquired an excellent reputation.

In 1904 the prison accommodated 81 men and 6 women prisoners.

The prison accommodated prisoners until 1916 when it was closed by the Prison Commissioners. Thirteen prison guards were transferred to Shrewsbury Prison. Acquired by the County Council for £4,000 in 1926, parts of the building became offices and a library. During World War II it was used as a munitions factory.