The first gaol at Ruthin of which historians have any knowledge, was situated in the old courthouse of the Lordship of Ruthin. Built in 1404, this half-timbered building in St Peter's Square was restored in 1926 and now houses a bank.

Until the 16th century, gaols were regarded mainly as a means of holding prisoners awaiting trial; the actual punishment inflicted varied from fines, through a spell in the stocks, to branding, whipping and execution. Executions at Ruthin generally took place in St Peter's Square. The last execution on the Square is believed to have been that of a Franciscan priest, Father Charles Mahoney (or Meehan), in 1679.

Towards the end of the 16th century prisons had come to be used as a means of punishment - for debt, non-payment of fines, minor misdemeanours and especially vagrancy. Offenders and unconvicted people (male and female) were usually crowded together in unpleasant and unhealthy conditions. Small gaols, like the one at Ruthin, proved unable to cope. To counter this an Act of 1576 enabled Houses of Correction to be built. The main purpose of the House of Correction, also known as the "Bridewell", was not to punish criminals but to eliminate vagrancy, which had become a problem throughout the county. Able-bodied idlers and the unemployed, instead of being thrown into prison, were sentenced to the Houses of Correction.

The judges at the Court of Great Sessions ordered the Denbighshire justices of the peace to build the first county House of Correction at Ruthin in 1654. The House was built at the bottom of Clwyd Street on the same site as the present jail buildings. However conditions at the House of Correction deteriorated as it came to be used as a general place of detention. The Denbighshire justices now decided to build an entirely new prison. The architect Joseph Turner of Chester was appointed to design it in 1775. This building served as the county jail until 1866.

After the Prison Act of 1865 it was decided to extend the existing gaol and a new four-storey wing was built. When completed the new prison could accommodate about 100 prisoners.

In 1877, under a new Prison Act, control of local prisons passed to the Home Office. In 1878 Ruthin County Gaol became H.M. Prison Ruthin and remained open for 38 years until 1916. In 1926 Denbighshire County Council bought the buildings from the Prison Commissioners and converted them for office and library use. During the Second World War the Gaol was taken over as a munitions factory. Floors were inserted in the space between the galleries of the 1866 block. The County Council returned after the war.