In the tenth and eleventh centuries Norsemenn gradually settled in lowland Caithness. Along with Orkney Caithness became a valuable source of food supply for Norway, which in its whole extent contained barely as much fertile and level land as there is in Caithness. Skilful seamen and capable shipbuilders, the Norse were well acquainted practically with the influence of sea power. Occupying a remote situation and separated by the Highlands from central Scotland, Caithness fell an easy prey to Norse vikings. The Jarl of Orkney, head of the Norse power in the British Isles, soon acquired control over Caithness and the east coast of what they called "Sudrland" (Southland). He then assumed the title of Jarl of Orkney and Caithness.
The Gaelic-speaking inhabitants meanwhile withdrew to the upland districts, where Celtic chiefs for centuries administered the customary Celtic laws. When the Norse were converted to Christianity in the eleventh century, their clergy were not at first brought fully into the organization of the Roman Church. The metropolitans of York and Hamburg contended for ecclesiastical jurisdiction over the Norse churches. About 1150 the country beyond the Dornoch Firth was formed into a diocese by David I of Scotland, The first bishop of the new diocese, named Andrew, had been a canon of Scone and enjoyed some reputation for learning. He had his "High Church" at Halkirk (High Kirk) on the borders of the Norse and Celtic parts of his diocese, In 1198 his successor, Bishop John, was savagely attacked by a follower of Jarl Harold and subjected to barbarous mutilation. Such treatment of a church dignitary aroused the indignation of the country. William the Lyon invaded Caithness in 1202 and compelled the Norse Jarl Harold Maddadson to make submission and to deliver up his daughter Matilda as a hostage for his good behaviour. It is doubtful whether the kings of Norway claimed any jurisdiction on the mainland after this incident, though King Hacon levied tribute upon Caithness in 1263. From the beginning of the thirteenth century the jurisdiction of the Jarls of Orkney and Caithness was limited to the north-eastern or lowland part of the diocese.
About 1220 another bishop of Caithness was subjected to outrage. Owing to a dispute about church dues Bishop Adam was seized by the people and burned to death in his own kitchen at Halkirk. In consequence of so serious an offence, a punitive expedition to Caithness was undertaken by Alexander II. Jarl John Haraldson, who had been party to the crime, was severely punished, and it is said that the Scottish king cut off the hands and legs of eighty men concerned in the murder of the bishop. A man of great administrative capacity, equally noted for his piety and his culture, Gilbert, Archdeacon of Moray, was appointed bishop of Caithness. Under him (1223-45) the cathedral chapter was constituted and the whole diocese divided into parishes. The parish endowments of Sutherland and Caithness were allocated to the various dignitaries and canons. The prebend of Thurso, Halkirk, and some other parishes was assigned to the bishops, who enjoyed these endowments until the abolition of episcopacy, when they lapsed to the crown, which in consequence owns valuable domains in Caithness to this day.
On the death of Jarl John Haraldson, in 1231, without an heir, the direct line of the Norse Jarls became extinct. The Scottish king conferred the earldom of
Caithness upon Magnus, son of the Earl of Angus and probably son or husband of Matilda, daughter of Jarl Harald. Magnus was at the same time Jarl of Orkney under the king of Norway, and Earl of Caithness under the king of Scotland. The Angus line of the earls of Caithness became extinct in 1329 and the earldom was then conferred, first on a branch of the Stewarts, and thereafter on the Crichton family. The Crichtons became extinct about the middle of the fifteenth century.
In 1455 James II granted the earldom of Caithness to William Sinclair, second son of the Jarl of Orkney, who was chancellor of Scotland and one of the leading Scottish statesmen of the period. The earldom has continued in the Sinclair family to the present clay, although the estates no longer go with the title. The present Earl of Caithness owns estates in Aberdeenshire.
The advent of the Sinclairs marked a period of much unrest and strife in the county due to feudal rivalries.
After the death of Alexander III Scotland continued for centuries in the grip of feudalism, and owing to the weakness of the government little progress was made by the people, who were kept in continual turmoil by the rivalries of local magnates. In 1426 there was a skirmish at Harpsdale between the Gunns (a Norse clan) and the Mackays of Strathnaver. Again in 1438 the Gunns and the Keiths fought on the moor of Tannach, near Wick. These clan contests continued until near the end of the seventeenth century. In 1667 the Mackays raided Caithness. In the following year the Sinclairs raided Strathnaver and took a "creach" of nine hundred cattle. In 1680 Lord Glenorchy, who had purchased the lands and title of the earldom from George, Earl of Caithness, invaded Caithness with an army of 700 Campbells to make good his claims. He was opposed by Sinclair of Keiss, who claimed to be the next heir to the earldom, and a fight took place at Altimarlach, two miles west of Wick, where the Sinclairs, partly by stratagem, were completely defeated. This was the last clan battle fought in Scotland. Glenorchy's piper, Findlay MacIver, composed at this time the well-known marching tune, "The Campbells are Coming". The government eventually recognized the claim of George Sinclair to the earldom, and Glenorchy, by way of recompense, received the title of Earl of Breadalbane. Glenorchy had found it impossible to collect his rents in Caithness. The people regarded him as an intruder and a usurper. The lands of the earldom passed into the possession of the Sinclairs of Ulbster, who also acquired the heritable jurisdiction. Some years afterwards Sir John Sinclair of Ulbster entered upon those improvements of the agriculture of the county with which the modern period of industrial progress began.
Other landowners followed suit and there was inaugurated an era of progress which has continued to the present day.