Spanish soldier-politician who played a prominent role in the successful Spanish military insurrections of 1843 and 1854 and headed the Spanish government three times between 1856 and 1866. Though he lacked a coherent political program, he was a staunch supporter of Queen Isabella II (reigned 1833–68) and pursued conservative policies while in office.

A descendant of the O'Donnells who left Ireland after the Battle of the Boyne (1690), O'Donnell gained fame and high rank during the First Carlist War (1833–39). He went into exile in France in 1840 but returned three years later to help overthrow the government of Gen. Baldomero Espartero (Espartero, Baldomero, príncipe de Vergara). He was rewarded with the captain generalship of Cuba (1844–48). In 1854, espousing liberal sentiments, he headed a successful military revolt that brought him to power as minister of war. In this post he shared control of affairs with Espartero, whom he displaced as premier in July 1856. His first administration lasted only until October, but, as head of the moderately conservative Liberal Union, he again held power from 1858 to 1863.

This politically stable period, during which O'Donnell governed under a conservative constitution of 1845, witnessed economic expansion, a compromise solution to the problem of church property, and O'Donnell's opportunistic interventions in Morocco, Santo Domingo, and Mexico. In the Moroccan War (1859–60) he himself commanded the victorious expeditionary force and greatly increased his popularity, afterward receiving his dukedom. O'Donnell resumed office as premier briefly in 1865–66. He at first attempted a conciliatory policy but had to suppress two insurrections in 1866, and Queen Isabella replaced him with the more repressive Gen. Ramón Narváez. His death not long after his dismissal deprived the Queen of one of her strongest allies, and a year later Isabella was deposed.