After fellow Catholic James II of England was deposed by the Glorious Revolution in November 1688, Sarsfield served as a senior commander in the Jacobite army during the 1689 to 1691 Williamite War in Ireland, and was elected to the 1689 Patriot Parliament.

This was emphasised by 19th-century writers seeking to bolster Sarsfield's status as an Irish hero, although nationalist historian O'Callaghan claimed he was "no better than a puffed Palesman", compared to figures such as Eoghan Ruadh Ó Néill.

His eldest son William married Mary Crofts, reputedly an illegitimate daughter of Charles II and younger sister of James Scott, Duke of Monmouth.

[4] This left him short of money, and Sarsfield became involved in an expensive legal campaign to regain Lucan Manor from the heirs of his brother William, who died in 1675.

This ultimately proved unsuccessful amid allegations of forged documents, and in 1681 he returned to London, where he made two separate attempts to abduct an heiress and was lucky to escape prosecution.

[13] When Charles's Catholic brother James became king in 1685, Sarsfield rejoined the army and fought in the decisive Battle of Sedgemoor, which ended the Monmouth Rebellion.

Aware of preparations for invasion by his nephew and son-in-law William of Orange, James sent Sarsfield to Dublin in September to persuade Tyrconnell to provide him with Irish troops.

[19] James returned to France, leaving Tyrconnell in control; he was the leader of the "Peace Party", who wanted to negotiate a settlement preserving Catholic rights to land and public office.

[15] Sarsfield headed the "War Party", who felt they could gain more by fighting on; it included the Luttrell brothers, Nicholas Purcell and English Catholic William Dorrington, a former colleague from Monmouth's Regiment.

[23] Tyrconnell returned in January 1691, carrying letters from James making Sarsfield Earl of Lucan, an attempt to placate an "increasingly influential and troublesome figure".

[27] However, the collapse of the Shannon line and surrender of Galway and Sligo left him little option; without French supplies, the military position was hopeless, and defections meant his army was dissolving.

[25] The military articles of the Treaty of Limerick preserved the Jacobite army by allowing its remaining troops to enter French service; about 19,000 officers and men, including Sarsfield, chose to leave in what is known as the Flight of the Wild Geese.

Despite several searches, no grave or burial record has been found, although a plaque at St Martin's church, Huy, has been set up in commemoration and an announcement in 2023 stated that, pending exhumation and identification, his remains had been located.

[31][32] He has also been criticised for the role he played in creating divisions within the Jacobite camp; senior officers considered him rash and easily manipulated, although he seems to have been popular with the rank and file.

[27] Mythologising began during his lifetime; the poet Dáibhí Ó Bruadair, 1625 to 1698, composed a panegyric describing Sarsfield as virtuous, heroic, popular and a great leader, but admitted they had not met.

[37] The global Irish diaspora meant his name and reputation were commemorated beyond Ireland; Michael Corcoran, a Federal general in the United States Civil War, claimed to be a direct descendant.