His grandfather, Rory O'More, was a leader of an Irish Catholic uprising against the English in 1641. Sarsfield himself served in the army of King Louis XIV of France from 1671 to 1678, and, after the accession of the Catholic James II to the English throne in 1685, he helped James's commander in Ireland, Richard Talbot (later earl of Tyrconnell), purge Protestants from the Irish Army. When James was deposed by the Protestant William of Orange (later King William III) in 1688, Sarsfield accompanied James to France and then to Ireland.

During the early months of the Irish Jacobite war against William, Sarsfield distinguished himself as a cavalry commander and was promoted to major general. On July 1, 1690, William severely defeated the Jacobites in the Battle of the Boyne. Sarsfield rallied the beaten army, organized the defense of Limerick, and in August made a spectacularly successful attack on William's artillery train at Ballyneety. He negotiated the final Jacobite surrender at Limerick (October 1691) and then joined Louis XIV's army in the Spanish Netherlands, where he was mortally wounded fighting the English at Neerwinden, near Landen, on Aug. 19, 1693. James had made Sarsfield earl of Lucan in 1691, but the title was recognized only by the Jacobites.