The first Earl of Caithness of the Sinclair line received in 1455, besides his earldom, a grant of the justiciary and sheriffdom of Caithness. In 1503 the Scots Parliament enacted that, owing to the distance of the northern parts of the sheriffdom from Inverness, courts might be held at Dornoch and Wick but the jurisdiction of the sheriff of Inverness was expressly reserved.
Important cases, such as the infeftment or service of the leading nobles, were still taken at Inverness. Sir Robert Gordon's success in getting Sutherland made a shire in 1631 seems to have stimulated the Earl of Caithness to secure similar privileges for his own county. In 1641 Parliament granted a Ratification to the town of Wick, declaring it to be the head burgh of the sheriffdom of Caithness. The Earl of Caithness was nominated sheriff of the new shire, and towards the end of the seventeenth century this heritable jurisdiction was sold to Sinclair of Ulbster. At the date of the abolition of these jurisdictions in 1747 a sum of £5000 was paid to Sinclair of Ulbster in compensation for the loss of the heritable jurisdiction of Caithness. The sheriff depute (known after 1829 as the sheriff principal) was formerly appointed by the heritable sheriff, hut the appointment now belongs to the crown.
In 1853 the counties of Sutherland and Caithness were united into one sheriffdom. In 1870 Caithness was detached from Sutherland and joined to Orkney and Shetland, while Sutherland was united with Ross and Cromarty.
In the administration of public health and education it would seem that ancient relations between Caithness and Sutherland bid fair to be revived, and the two counties were in 1918 united into one parliamentary constituency.
The parishes of the county, nearly all created in the time of Bishop Gilbert (1223-45), have for seven centuries formed the limits of the local ecclesiastical jurisdictions. The county contains ten parishes constituting the presbytery of Caithness. The two presbyteries of Dornoch and Tongue in Sutherland and the presbytery of Caithness form the Synod of Sutherland and Caithness, which is co-extensive with the ancient bishopric of Caithness. The parishes form the areas for Parish Councils, created in 1894.
Education is administered by the County Education Authority, created in 1918. The Higher Grade Schools in Wick and Thurso are practically secondary schools, while those at Halkirk, Castletown, and Lybster prepare pupils up to the intermediate stage.
Soon after the erection of Caithness into a shire in 1641, Commissioners of the shire were appointed to provide supplies of men and equipment for the Scots Army which fought against Charles I. These Commissioners of Supply became the administrative authority for the county and continued to be so until the creation of County Councils by the Local Government (Scotland) Act of 1889. The chief executive officers of the county are the Lord-Lieutenant-head of the Court of Lieutenancy-and the Sheriff. Under the Disarming Acts passed in 1716 and 1724 and other older statutes the Lord-Lieutenant possessed a certain military jurisdiction, but in 1907 the County Territorial Force Association took over the military administration of the territorial forces within the county. Prior to the war these forces in Caithness were attached to the Seaforth Highlanders, with headquarters for the regulars at Fort George and for territorials at Golspie.
The administration of the police of the county is vested in a Joint Committee appointed by the County Council and the Commissioners of Supply. The County Road Board consists of members of the County Council and representatives of the Parish Councils. Public Health is administered by the County Council in the county and by the Town Council in Wick, the only royal burgh in Caithness. Thurso, a burgh of barony, possesses its own burghal administration for general purposes. The administration of state insurance is vested in a County Insurance Committee. Under the Licensing Acts there is a County Licensing Court, composed partly of county councillors and partly of justices of the peace.
In the administration of mental deficiency Caithness is associated with a southern District Board instead of with the neighbouring counties of Inverness, Ross and Sutherland. For the purposes of agricultural education the county is included in the district of the North of Scotland College of Agriculture.