Justiciar is the English form of the medieval Latin term justiciarius or justitiarius (meaning "judge" or "justice"). The Chief Justiciar was the king's chief minister, roughly equivalent to a modern Prime Minister of the United Kingdom.

The Justiciar of Ireland was an office established during the Anglo-Norman invasion of Ireland and was a key tool in its colonisation. Following the conquest of the Principality of Wales in the 13th century, the areas that became personal fiefs of the English monarchs were placed under the control of the Justiciar of North Wales and the Justiciar of South Wales.

A similar office was formed in Scotland, although there were usually two or three – the Justiciar of Scotia, the Justiciar of Lothian and, in the 13th century, the Justiciar of Galloway. These offices later evolved into a national one called Lord Justice-General. The modern title is Lord President of the Court of Session.

Similar positions existed in continental Europe, particularly in Norman Italy and in Sweden.

In Norman England, kings enlarged the scope of royal justice by delegating judicial authority to members of the curia regis (Latin for "king's court"). These were called justiciars. Henry I (r. 1100–1135) appointed local justiciars to supervise a county or group of counties. It was not until the reign of Henry II (1154–1189) that the title was exclusively applied to the king's chief minister.

After the Norman Conquest of 1066, the King of England was also Duke of Normandy and divided his time between the two territories. In his absence, William the Conqueror (r. 1066–1087) temporarily delegated viceroyal authority to trusted officers described variously as regent, custodian, and prefect. When William Rufus (r. 1087–1100) became king, this temporary role developed into a more permanent and defined office. Rufus entrusted the control of government administration to his chaplain, Bishop Ranulf Flambard of Durham. Flambard ran the government at all times, even when Rufus lived in England. Historian Frank Barlow argues that Flambard was the first chief justiciar. While Flambard was probably the first to exercise the powers of a chief justiciar, he never held that rank officially. Nevertheless, he was described by contemporary chronicler Orderic Vitalis as a justiciarius.

Sometime around 1107 or 1108, Henry I appointed his chancellor, Roger of Salisbury, as the first chief justiciar. Roger was described by chroniclers as secundus a rege (Latin for "second from the king"). Roger oversaw the administration of justice, ecclesiastical appointments, and royal finances. According to the chronicler Symeon of Durham, Roger made most decisions for the royal government. Historian Bryce Lyon writes that "Roger was a sort of medieval prime minister but a minister immeasurably more powerful because his only responsibility was to his lord the king."

The chief justiciar was responsible for directing the royal household, the curia regis, and the government departments. Nevertheless, he was not a member of the household. He was the presiding officer of the exchequer and directed the procedures of the curia regis as the chief royal justice. He also supervised the itinerant justices.