He began his active military service during the Williamite War in Ireland in 1689 and ended it with the suppression of the 1715 Jacobite Rebellion.

A close associate and confidant of the Duke of Marlborough, he was also a diplomat and Whig politician who sat in the English and British House of Commons from 1705 until 1716, when he was raised to the peerage as Baron Cadogan.

A strong supporter of the Hanoverian Succession, he took part in the suppression of the 1715 Jacobite Rebellion and succeeded Marlborough in 1722 as Master-General of the Ordnance and senior army commander.

He was one of five children, including two brothers Ambrose and Charles and two sisters: Frances, who died young, and Penelope, who married Sir Thomas Prendergast, 1st Baronet.

William's father intended him to take up a law career like himself and, in March 1687, he was accepted as a student at Trinity College, Dublin.

[4] Midway through his studies, however, the Glorious Revolution took place in England in which the Protestant William of Orange seized the throne from the Catholic James II.

In Ireland, the largely Catholic Irish Army remained loyal to James, while Protestants declared their support for William.

Following the relief of Derry and Enniskillen by a large expeditionary force under Percy Kirke, Cadogan served with the Williamite troops for the remainder of the Irish War.

[7] Following the climatic victory at the Siege of Limerick in 1691 he continued to serve in Ireland for three years having decided to become a professional soldier rather than return to his law studies.

[8] In June 1701, Cadogan was selected as Quartermaster General to Marlborough on the latter's appointment to command the British contingent in the Low Countries.

Cadogan was made Marlborough's chief of staff, soon becoming a trusted figure alongside other intimates including the General's brother Charles Churchill, military secretary Adam de Cardonnel and the artillery commander Colonel Holcroft Blood.

In early 1704 while travelling back to England, carrying important documents, his ship was attacked by a French Privateer.

[23] After Marlborough's dismissal from his posts at the end of 1711 Cadogan remained with the army, but refused to return with it when Britain withdrew from the war in 1712, going into voluntary exile with the Duke.

He was strongly opposed to the terms of the Treaty of Utrecht, agreed by the Tory Government, siding with the opposition Whigs who called for "No Peace Without Spain".

He was tasked with restoring the relationship with Britain's recent ally which had been damaged by the country's sudden withdrawal from the war.

Argyll as the senior Scottish commander led the initial attempts to contain the Jacobites from his position at Stirling Castle.

Cadogan established better supply lines for the Army, personally took part in scouting operations, and organised the advance on the rebel capital at Perth.

Cadogan's task was to oversee continued military operations across northern Scotland, forcing the leading Clan chiefs to submit.

On 21 June 1716, he was made Baron Cadogan of Reading, having recently purchased Caversham Park, Oxfordshire (now Berkshire) near that town.