The head of government within the principality of Wales was the Justiciar, the chief officer or deputy of the prince, responsible only to the prince, or to the king if there was no prince. Two Justiciarships existed, because of the administrative division of the principality. The Justiciar or justice of North Wales, an office established around 1284 under the Statute of Wales 1281 c3, was responsible for Anglesey, Caernarvon and Merioneth: the seat of government was at Caernarvon.
In the early years of the principality, the Justiciars had wide-ranging powers, and acted as the prince's personal representatives, each being the political, judicial and administrative head of princely government within his part of the principality. However, as Wales became more settled, the Justiciars became effectively restricted to the administration of justice, leaving the chamberlains of North and South Wales as senior officials.
The Justiciar of North Wales, or his deputy, held Great Sessions for each of his shires twice a year, for crown pleas, civil pleas, assizes and gaol delivery. These sessions were held quite regularly over the two and a half centuries of the principality, as far as can be judged from the few surviving records.
Between 1536 and 1543, a new judicial system was established through all of Wales, by the Laws in Wales Act 1535 c26 and the Laws in Wales Act 1542 c26. The marcher lordships were amalgamated into seven new shires, of which Monmouthshire was transferred to Westminster jurisdiction. The other six, together with the five old shires of the principality of Wales and Flintshire, were given a coherent system of justice by these two statutes. The 1542 statute codified developments that had taken place since 1536. The administration of justice in Wales was henceforward carried out according to a plan that combined English methods of local administration with a superior jurisdiction based on Welsh models.
Local administration was based on sheriffs, coroners and (an introduction new to Wales) justices of the peace. Above that level were four Courts of Great Sessions, modelled on the medieval jurisdiction of the Justiciar of the principality of North Wales and his courts of Great Sessions. The four new courts operated within four circuits, each with its own justice. The North Wales circuit was composed, like the old principality of North Wales, of Anglesey, Caernarvon and Merioneth. The Carmarthen circuit covered Carmarthen, Cardigan and Pembroke, and the Brecon circuit covered Brecon, Glamorgan and Radnor. Denbigh and Montgomery were joined by Flint in the Chester circuit: the two new shires were placed, as Flint had been in 1282, under the justice of Chester. In 1576, a second justice was added to each circuit. The jurisdiction of these justices within their circuit was extensive: it covered the entire range that in England was covered by the Courts of King's Bench and Common Pleas, and by the commissions of assize, gaol delivery, and oyer and terminer. From these Courts of Great Sessions, error lay to King's Bench in real or mixed actions, and to the Council in the Marches of Wales in personal actions.
The procedures to be followed by both local and circuit officials and justices were given in detail, in the 1542 statute. The justices of Great Sessions were to ride their circuits twice a year, sitting for six days at a time in each county town: fees, costs, legal process and official duties were all spelled out in the statute. The administrative support needed for the courts was also listed. Each circuit had two seals for the issue of writs: the original writs were issued by the chamberlain, head of an Exchequer or Chancery on the principality model. Judicial writs were issued by the prothonotary of each circuit, who also had the duty of entering the pleas, process and matter of record. The Exchequers on each circuit were the places of deposit for records, and also of account for the sheriffs: in practice, it appears that Great Sessions records were kept with the particular county of the circuit to which they appertained. Like the old principality Exchequers, these Exchequers had both a Chancery and Treasury function: they did not have equitable jurisdiction. From about 1689, however, the four Exchequers, by then often known as Chanceries, exercised an equitable jurisdiction that they apparently gained on the abolition of the Council in the Marches of Wales.
In 1830 the separate jurisdiction for Wales (and also that of Chester) was abolished by the Law Terms Act 1830 c70, and Wales was incorporated into the English system of central courts at Westminster and itinerant assize courts. Extra judges were appointed to King's Bench, Common Pleas and the Exchequer to cope with future Welsh pleas. The civil cases pending at Great Sessions in 1830 were transferred to Westminster: the common law suits to the Exchequer of Pleas, and equity suits to the Court of Chancery or to the equity side of the Exchequer. Within Wales the four circuits of Great Sessions were merged into two assize circuits: the North Wales Division and the South Wales Division.