The East York Militia was a part time home defence force in the East Riding of Yorkshire. The Militia and its predecessors had always been important in Yorkshire, and from its formal creation in 1759 the regiment served in home defence in all Britain's major wars until 1919. It became a battalion of the East Yorkshire Regiment, and its role during World War I was to train thousands of reinforcements for the regiment's battalions serving overseas.

The English Militia was descended from the Anglo-Saxon Fyrd, the military force raised from the able-bodied freemen of the shires under command of their Sheriffs. The three Ridings of Yorkshire and adjacent counties provided the bulk of the fyrdmen who fought against Harald Hardrada at the Battles of Fulford and Stamford Bridge in 1066. The Shire levy continued under the Norman and Plantagenet kings: Yorkshire levies helped to defeat the Scots army at the Battle of the Standard (1138). The Shire levy was reorganised under the Assizes of Arms of 1181 and 1252, and again by the Statute of Winchester of 1285. East Riding levies were regularly employed in offensive and defensive campaigns against Scotland, including the battles of Halidon Hill (1333), Neville's Cross (1346), Berwick (1482) and Flodden (1513). They were also seen in domestic conflicts such as the Wars of the Roses: the Mayor of Kingston upon Hull (Hull) was killed at the Battle of Wakefield in 1461 at the head of three companies of infantry raised in the town, and East Riding detachments were prominent in the Pilgrimage of Grace in 1536.

Under the Tudors the legal basis of the militia was updated by two acts of 1557 covering musters (4 & 5 Ph. & M. c. 3) and the maintenance of horses and armour (4 & 5 Ph. & M. c. 2), which placed the county militia under a Lord Lieutenant appointed by the monarch, assisted by the Deputy Lieutenants and Justices of the Peace. The entry into force of these Acts in 1558 is seen as the starting date for the organised county militia in England. Although the militia obligation was universal, it was clearly impractical to train and equip every able-bodied man, so after 1572 the practice was to select a proportion of men for the Trained Bands, who were mustered for regular drills. During the Armada campaign of 1588, the militia of Yorkshire were assigned to watch Scotland and the East Coast of England. The East Riding Trained Bands mustered 1600 men, of whom 640 were armed with calivers, 560 with pikes, 240 with bills and 160 with bows.

King Charles I attempted to bolster the Trained Bands as a national force under royal control. In the Bishops' Wars the East and North Yorkshire Trained Bands were expected to join the King's Army, with deficiencies in their arms made up from the arsenal at Hull, but there was great reluctance throughout the militia to serve outside their own counties, even for pay. Control of the militia was one of the areas of dispute between King Charles I and Parliament that led to the First English Civil War. On 12 February 1641 Parliament appointed the Earl of Essex as a fit person to be entrusted with organising the Trained Bands of Hull and the East Riding. When Charles approached Hull in April 1642, Parliament's governor, Sir John Hotham, called out 800 men of the Trained Bands (illegally, since the position of Lord Lieutenant was vacant) and prevented the king from seizing the arsenal. In response, Charles called out the remainder of the East Riding's men as a regiment under Sir Robert Strickland to join his investing army. In the event, both sides took the Trained Bands' weapons and gave them to paid volunteers who would serve anywhere in the kingdom in permanent regiments.

On the Restoration of the Stuarts the Militia was reorganised (though the term 'Trained Band' endured in East Yorkshire until the end of the 17th century). In 1689 the East Riding's contingent consisted of one regiment of foot commanded by the Marquis of Carmarthen as Lord Lieutenant, made up of eight companies with a total strength of 679, and two 64-man Troops of cavalry. In the 1697 returns, the East Riding had one regiment of eight companies of foot totalling 679 men, and two troops of horse with 128 men, under Colonel the Marquis of Carmarthen (son of the Lord Lieutenant, who had been elevated to Duke of Leeds). When mustered they 'appeared to be in very good order'. At the time of the Jacobite rising of 1715 the East Riding Regiment of Militia consisted of about 670 men under the command of Lieutenant-Colonel Robert Hildyard. Thereafter the militia was allowed to decline, although when Viscount Irwin became Lord Lieutenant of the East Riding in 1738 he reviewed the state of the militia and began to appoint new officers. Nevertheless when the Jacobite rising of 1745 began he advised against calling out the inefficient militia and instead enrolled volunteer companies for home defence.

Under threat of French invasion during the Seven Years' War a series of Militia Acts from 1757 re-established county militia regiments, the men being conscripted by means of parish ballots (paid substitutes were permitted) to serve for three years. There was a property qualification for officers, who were commissioned by the Lord Lieutenant. Opposition to the ballot led to rioting in some counties, including the East Riding, and organisation of the new force proceeded slowly. In the East Riding the first issue of arms was only made on 3 December 1759; the regiment was embodied at Beverley for service on 8 January 1760 and marched off to Newcastle upon Tyne. The East York Regiment of Militia comprised 33 officers and 460 other ranks, organised into 10 companies, under the command of Colonel Sir Digby Legard, 5th Baronet. The men were known locally as the 'Beverley Buffs' or the 'Yorkshire Buffs' from the colour of their uniform's facings.

The militia was stood down ('disembodied') at the end of the war, after which their obligation was for 28 days' training each year, but this was often neglected. The East York Militia was embodied again on 3 March 1778 during the War of American Independence when Britain was threatened with invasion by the Americans' allies, France and Spain. The East York Regiment, commanded by Henry Maister, was complimented on its appearance and drill when it was inspected at York before marching south. The East York Militia was at Warley Camp in Essex in the summer of 1778 and the following summer at Coxheath Camp near Maidstone in Kent. At these large training camps the Militia were exercised as part of a division alongside Regular troops while providing a reserve in case of French invasion of South East England.The East Yorks along with the North Yorks Militia formed part of the Left Wing at Coxheath. Each battalion had two small field-pieces or 'battalion guns' attached to it, manned by men of the regiment instructed by a Royal Artillery sergeant and two gunners. The regiment was disembodied in March 1783.

Militia training was again neglected during the subsequent peace, but the regiments were embodied for almost continuous service during the French Revolutionary and Napoleonic wars of 1793–1815. On 28 December 1792 the East York Militia was reviewed by the Lord Lieutenant in Beverley Minster (its normal place for parades in the winter) and on 31 December the regiment was embodied, still under Henry Maister (who would remain in command until 1803). In February 1793 the regiment marched to King's Lynn where it was employed in coast defence. Under the Militia Act 1794 a further two companies were raised for the regiment in 1795.