He inherited titles in the Jacobite and Spanish nobility on the death of his father in battle in 1734 at Philippsburg, during the War of the Polish Succession.

Sarsfield and Berwick were both veterans of James II's Irish Army and went into continental exile as part of the Flight of the Wild Geese.

On 31 December 1716, the 2nd Duke of Berwick became the second husband of the dowager Catalina Ventura Colón de Portugal y Ayala-Toledo, 9th Duchess of Veragua (14 July 1690 – 3 October 1739).

They had 6 children, of whom 4 survived to adulthood:[1] The Anglo-Franco-Spanish James Fitz-James Stuart was employed by Elizabeth of Parma to achieve her military purposes, being involved in the battles on land and sea from 1717 to 1719 to regain Naples from its Austrian rulers and Sicily from the Duchy of Savoy, thus creating the Kingdom of the Two Sicilies.

He fought as colonel of the Irish Regiments of Spain and Lieutenant General of the Spanish Royal Armies, being promoted to Field Marshal in February 1724.