Guernsey, like the other Channel Islands was under constant threat of invasion or attack. As early as 1061, prior to the invasion of England, William, Duke of Normandy sent soldiers to Guernsey under his Squire Sampson d'Anneville to protect the Island from pirate raids. Indeed, a number of Notable Island names fought with William in 1066 when he invaded England.

On 24th July 1203 King John ordered that Island Seigneurs were to provide "sufficiency of men and money to defend the Island from the enemy…" These were under the command of Reginald de Carteret. The first definitive mention of the Militia, however, was not until 1336 when after an invasion attempt by David Bruce, the exiled King of Scotland (son of Robert Bruce). In 1337 Edward III commissioned Thomas de Ferres to "levy and train the inhabitants of Guernsey, Jersey, Sark and Alderney, to the use of arms and to array them in millenis, centenis and vingtenis (thousands, hundreds and twenties). The following year this new force was put to the test when a French Admiral by the name of Behuchet landed with an invasion force on the west coast, men of the St Martin's and Forest Militia challenged the French at Les Hubits and a battle ensued. The Militia repelled the invasion force and as a result the South Regiment of Militia were granted the right to wear blue facings on their uniforms, (normally a privilege granted only to Royal Regiments) by Charles II nearly three centuries later.

The Militia evolved over the next century on a Parochial basis, with each Parish raising a company of soldiers commanded by a Captain, These companies normally consisted of 100 men aged between 16 and 60. There was one company per Parish with the exception of St Peter Port which had four. By 1621 the number of serving Militiamen was 1157, in 1656 there were 1418 and by 1680, 1902 men were serving in the Guernsey Militia.

Guernsey was staunchly Parliamentarian during the English Civil War. The Lieutenant Governor, Sir Peter Osborne, however, was a Royalist and fled to Castle Cornet where he took refuge and held out with assistance from Jersey. In 1651 the Guernsey Militia attacked the Castle, however, the attack was unsuccessful, as Royalist supporters from Jersey had reinforced the Castle Garrison. Later that year the Castle was evacuated when the Garrison was paid an indemnity and allowed to leave with full military honours.

By the end of the 17th Century the Guernsey Militia consisted of 13 companies, totalling 1600 men. The first Regiments were formed in 1711 and were named as follows:

1st (Town) Regiment - made up from St Peter Port and St Sampsons
2nd (North) Regiment - made up from Catel and Vale

The 3rd (South) Regiment - made up from St Martin's, St Andrew's, Forest, St Saviour, St Peter in the Wood and Torteval was formed in 1743. A Troop of Horse was raised in 1735; the intention being that the unit would provide messengers to carry orders and information between the various posts across the Island. The Guernsey Militia was re-organised in 1755, two companies of Artillery were formed and the St Sampson's company was transferred from the Town regiment to the North Regiment. The 4th (West) Regiment was formed around 1780 when St Saviour, St Peter in the Wood and Torteval were transferred from the South Regiment.

The British Government provided all weapons, equipment and accoutrements after 1782 and the colour facings on their uniform differentiated the Militia Infantry Regiments. The Town Regiment wore Buff facings with Gold lace, the North Regiment Green facings with Gold, the South Regiment Blue facings with Silver and the West Regiment Black facings with Silver lace. In 1783 gunners of the Town Regiment were involved in putting down a mutiny by 600 men of the 104th Regiment of Foot.

A further re-organisation of the Militia took place in 1803 by the Lieutenant Governor Sir John Doyle. The North, South and West Regiments became Light Infantry and were renumbered accordingly. The Guernsey Militia now consisted of:

Three Light Infantry Regiments
One Foot Regiment
A Troop of Light Dragoons (descending from the earlier Troop of Horse)
An Artillery Regiment
A Pioneer Corps

In 1804 men from the Town Regiment were used to man the guns between Fermain Bay and Bec du Nez this section was known as the Fermain Corps. A Brigade of Royal Marines (also known as the Sea Fencibles or Naval Brigade) was also raised on 16th August 1810, however, the function of this unit is unclear and it was subsequently disbanded in 1860. A second Artillery Regiment was formed in 1822 to man the Islands coastal defences and the Fermain Corps was absorbed into this Regiment. It was, however, short-lived and was disbanded in 1833 and the respective Parish Regiments manned the coastal defences. In 1825 the Light companies which formed part of each Infantry Regiment were converted to Rifle companies and as such wore green uniforms with black belts and pouches as opposed to red with white.

Another significant change in uniform came in 1831 when the whole of the Channel Islands Militia was made Royal on the occasion of the 50th Anniversary of the Battle of Jersey. All the facings on the Militia uniforms were now blue. Over the next 50 years various changes took place in the Royal Guernsey Militia. In 1834 the Town Regiment also became a Light Infantry Regiment and the following year, 1835, the Royal Guernsey Light Dragoons was disbanded. Five years later in 1840 the upper age limit was reduced from 60 to 40. In 1858 the Grenadier and Light Companies were disbanded. The Royal Guernsey Militia Rifle companies were disbanded in 1870, which caused much resentment as the riflemen considered themselves as the elite and were unhappy at having to return to the red tunics. In 1878 the West Regiment was disbanded and the men transferred to the South Regiment.

One of the most significant events in the History of the Royal Guernsey Militia occurred in 1896 when the North Regiment mutinied. The Commanding Officer of the Battalion retired that year and as there was no one qualified to take command the Senior Medical Officer Lieutenant Colonel Joseph Leale took control but without the assistance of a Second in Command, as a result the morale and discipline of the Battalion plummeted. When Captain John Beville Pym of the Royal Marines was appointed Adjutant he tried to instil some discipline but was ignored by the soldiers and even some Junior Officers. They refused to parade for the Adjutant and a number of men were imprisoned for mutiny. As a result of this mutiny unrest spread to the other Battalions and the whole of the Royal Guernsey Light Infantry was suspended pending an investigation and recommendations by the War Office. The Royal Guernsey Light Infantry did not parade again until 1901 when it was subject to the Army Act and some significant changes had been made. The mutinous North Regiment had been disbanded and the Royal Guernsey Militia consisted of:

1st Light Infantry Battalion - recruited from Town area
2nd Light Infantry Battalion - recruited from country districts
3rd Light Infantry Battalion - consisted of men formerly from the old South Regiment

The Royal Guernsey Artillery was supplemented with two Engineer companies, a field artillery company and a Cadet Company made up of boys from Elizabeth College. All these companies became known as the Royal Guernsey Artillery and Engineers (RGA&E). The Cadet Company was later detached from the RGA&E to become an Officer Training Corps.

When the First World War was declared in 1914 the Royal Guernsey Militia was immediately mobilised. The States agreed to send a contingent to help the war effort, Infantrymen from the Royal Guernsey Light Infantry (RGLI) formed two companies, one in the 6th Battalion Royal Irish Regiment and the other in the 7th Battalion Royal Irish Fusiliers. In November 1916 the Royal Guernsey Militia was suspended for the duration of the war and the RGLI was reformed as part of the British Army. The 1st (Service) Battalion serving overseas and the 2nd (Reserve) Battalion remained in the Island to receive and train recruits before they were despatched to the 1st Battalion for service overseas. In addition to the RGLI, the Officers and men of the RGA&E were formed into the 9th Divisional Ammunition Column, which was mobilised on 7th May 1915 and sent to France. The Quarrymen from Guernsey were assembled to form the 321 Quarrying Company Royal Engineers, they arrived in France on 15th February 1917.

In 1918 soldiers of the Royal Engineers Company were transferred to a new unit, the 245 (Guernsey) Army Troop Company Royal Engineers, they were joined by men from the 1st RGLI and the Royal Alderney Militia these proceeded to France on 10th April 1918. In 1916 Militiamen who were not covered by the Conscription Act due to their age were recruited into the Guernsey Volunteer Corps, very little is known about this unit.

All the Guernsey Officers in the 6th Royal Irish Rifles and the 7th Royal Irish Fusiliers were transferred to the 1st RGLI upon its formation. The remaining gunners of the RGA&E reformed into 109 / 110 companies of the Royal Garrison Artillery and manned the coastal defences of the Island. Sappers made up 166 Company of the Royal Engineers, these units, however, did not leave Guernsey.

On 14th January 1916 D Company of the 6th Royal Irish Rifles went into the line and on 26th January Guernsey suffered its first casualties from enemy action. The Service Battalion joined the 29th Division and commenced training for the Battle of Cambrai. This attack which took place on 20th November 1917 was a huge success, 86 Brigade of which the RGLI were part was tasked with the capture of Bois des Neufs (Nine Wood), the Brigade advanced up the west slope with the Middlesex Regiment in the lead and the RGLI on the left. The Middlesex Regiment was held up by resistance but the RGLI swept around the left flank and took the quarry that was in the wood. Later when the Germans counter attacked, 86 Brigade were redeployed to meet the threat and the RGLI were ordered to hold the little town of Les Rues Vertes, south of the river. Whilst in the town the RGLI were initially pushed back but they retook the village twice in heavy and vicious hand to hand fighting. Due to heavy losses the RGLI were pulled back and reformed.

On 18th January 1918 they were sent back into the line just beyond Ypres. They remained in the Passchendaele area until April 1918 and were then posted to the Lys area south of Bailleul in the North of France. The RGLI had withdrawn to a few hundred yards east of Doulieu, however, Doulieu was bypassed by the Germans and the Battalion was ordered to withdraw west and dig in facing south, they later had to withdraw a further thousand yards and by this time casualties were very heavy. On 14th April 1918 the 1st RGLI were relieved by Australian troops, as a fighting unit the RGLI was finished.

On 21st May 1919 the Service Battalion of the RGLI sailed on the SS Lydia, their destination was Guernsey; the Battalion was disbanded shortly afterwards. The 1st Battalion was granted the following Battle Honours, Ypres 1916, Passchendaele, Cambrai 1917, Lys, Estaires, Hazebrouck and France and Flanders 1917-1918.

In 1920 attempts were made to restart the Militia. The Second Battalion was disbanded in 1921 and the RGA&E was supplemented by a Royal Army Medical Corps section. In 1928 the British Government withdrew financial support for the Militia and the RGLI and the RGA&E were disbanded, however, in 1929 the States of Guernsey agreed to fund a much-reduced force of 250 men and the Royal Guernsey Militia was reformed.

The Royal Guernsey Militia was mobilised again in 1939 just before the outbreak of the Second World War. In January 1940, however, the Militia was suspended once again for the duration of the war. The men were given the option of either returning to civilian life or joining the regular forces. Many of the Officers and men formerly of the Royal Guernsey Militia served with great distinction with the British Army throughout the Second World War. The Royal Guernsey Militia Association still meet regularly, although their numbers are sadly dwindling year by year.