As part of the larger London Trained Bands, they were periodically embodied for home defence, such as during the 1588 Spanish Armada campaign.

Although service was technically restricted to London, the Trained Bands formed a major portion of the Parliamentarian army in the early years of the First English Civil War.

After the New Model Army was established in April 1645, they returned to their primary function of providing security for the palaces of Westminster and Whitehall.

Following the 1660 Stuart Restoration, the City of London Militia Act 1662 brought them under the direct control of the Crown, with the Trained Bands becoming part of the British Army.

The English militia was descended from the Anglo-Saxon Fyrd, the military force raised from the freemen of the shires under command of their Sheriff.

[16][23] Control of the trained bands was one of the major points of dispute between Charles I and Parliament that led to the English Civil War.

[24][25] There is an often-repeated story that when Charles I returned from his Scottish campaign in October 1641 he ordered the guards on Parliament sitting at Westminster, which were provided by the city, Surrey and Middlesex TBs under command of the Puritan Robert Devereux, 3rd Earl of Essex, to be replaced by the Westminster Trained Bands (many of whose tradesmen members were purveyors to the Royal Court at Whitehall Palace) under the command of the Royalist Lord Lieutenant of Middlesex, the Earl of Dorset.

There was still suspicion of the Westminsters' commitment, they took their turns at guard duty and later on campaign in defence of Parliament, although their numbers were often few and defaulters numerous.

[30] When open was broke out between the King and Parliament, neither side made much use of the trained bands beyond securing the county armouries for their own full-time troops.

[33] The main exception was the London area, where the large and well-trained regiments supported Parliament, beginning with the Battle of Turnham Green in November, where Charles was turned away from the city.

At the time of Turnham Green the citizens had erected breastworks across all the streets leading to open country and set up guard posts.

During the winter of 1642–3 volunteer work gangs of citizens constructed a massive entrenchment and rampart round the city and its suburbs, enclosing the whole of Westminster and the Tower Hamlets.

Not all could be called away at once – the need to man the defences and continue the economic life of the City precluded that – but during the active campaigning season the regiments took turns to do tours of duty in the field, receiving pay for a month.

[30][36] At this time the regiment retained the older system of 300-man companies, making it the strongest under the London Militia Committee with 2018 men in September 1643.

[43] One City Brigade took part on the Earl of Essex's relief of the Siege of Gloucester and the subsequent First Battle of Newbury on the march home.

Meanwhile, a great muster of the LTBs had been held in Finsbury Fields on 24 September and regiments were chosen by lot for a second brigade to join Sir William Waller's South Eastern Association army.

The projected move to Winchester was halted by snow and the force returned to the barns and farm buildings it had occupied the previous night.

Skirmishing continued around the outbuildings next day, but deputations from the London regiments asked Waller to be allowed to withdraw because of the bad weather, while the paid substitutes had run out of money.

Although their officers voted to fight Hopton's approaching army, the trained bandsmen refused (there had been rumours that they were to march to relieve the Siege of Plymouth).

The Royalists were taken by surprise as Waller's infantry assaulted the town, the London Brigade supported by the regular garrison of Farnham Castle attacking from the west.

The Westminsters and the Farnham Greencoats attacked a breastwork, whose defenders retired when outflanked by the Green Auxiliaries, allowing the brigade to enter the town.

With Oxford partially encircled, the King and the Royalist field army left the city and moved to Evesham, followed by Essex and Waller to Stow-on-the-Wold.

Next day the two armies marched parallel to each other on the high ground on either side of the river until Waller saw a gap opening in the Royalist line.

The Tower Hamlets TBs stoutly defended the west side of the bridge, preventing the Royalists from crossing to complete the destruction of Waller's army.

There was only skirmishing next day, but hearing that a reserve brigade of the City Auxiliaries was on its way, the King took the opportunity to break contact with Waller's battered force.

Essex and Waller worked round to attack from the west towards Speen village while Manchester's army remained to the east, using about 1000 skirmishers to distract attention from the pincer movement.

Despite Royalist reports that the London brigade was with Manchester, suffering heavy casualties in his skirmish line and final attack, they were in fact with Essex's army, which had made a 13 miles (21 km) march to get into position.

The revived London Militia Committee demolished the Lines of Communication and returned the suburban TBs to local control.

[100] During the Worcester campaign of the Third English Civil War in 1651, the Middlesex Militia were ordered to rendezvous at St Albans while the LTBs remained guarding London.