The term 'Militia' is defined as "a body of citizens enrolled as regular military forces for periodical instruction, discipline and drill but not called upon for active service except in emergency". In 1337 King Edward III sent a Commission to levy and train the inhabitants of Jersey, Guernsey, Alderney and Sark. Forty men for the defence of Sark were established by Helier de Carteret and were known as the Quarantaine. The Quarantaine was therefore the original Militia in Sark.

The Seigneur of Sark was traditionally Commander of the Militia consisting of an Infantry Unit, which also manned the Island's guns. Sark was part of the Bailiwick of Guernsey and its defences were under the general command of the Governor of Guernsey.

In 1642 during the English Civil War Jersey remained loyal to the King but Guernsey and consequently Sark was Parliamentarian. In 1644 Captain Lane of the Jersey Militia attempted to recapture Sark from Guernsey and the Parliamentarians. Although they were defeated in May 1644 the States of Guernsey ordered that there should be a permanent garrison on Sark to prevent any future incursions.

By 1744 all men between 16 and 60 had to serve in the Militia and keep watch against invasion and with this the Militia grew to a force of about 100 men. The upper age limit was later reduced to 45. In 1793 cannon were sent to Sark from Guernsey to be mounted on the headlands and barracks were built on Little Sark to house British Troops stationed on the island to reinforce the Militia, to guard against possible invasion by the French during the Napoleonic Wars.

The Sark Militia was listed in 1805 as a Light Infantry Unit and in 1831 was honoured, along with the other Channel Islands Militias, with the designation of 'Royal'. At this time the uniforms were changed from Red with Yellow facings to Red with Blue facings and Silver lace. After 1881 the facings became scarlet laced with Gold. The Sark Militia Band wore a white uniform with Scarlet facings.

In 1869 the force consisted of 68 men and a further 23 on reserve these forming two companies, one infantry and one artillery. The annual cost of the Royal Sark Militia to the British Government was £123 and £3 to the Island. The Royal Sark Militia was disbanded around 1875 although it was listed by the War Office until 1900. During the First World War many men from Sark served overseas with the Royal Guernsey Light Infantry.