It was founded by Leopoldo O'Donnell in 1858 with the intent of forging a compromise and taking a centrist position between the two forces that had hitherto dominated Spanish politics during the reign of Isabella II.
O'Donnell's intent was to bring together the non-absolutist Moderates and the less "exalted" (radical) Progressives and to occupy the political center.
After the elections to the Cortes 20 September 1858, the Liberal Union had a majority of seats; it maintained power continuously until the Glorious Revolution of 1868 ushered in the Sexenio Democrático.
The Liberal Union centered on O'Donnell and the iron hand of his Minister of Governance, José Posada Herrera.
These people, like the party's supporters in general, came from a broad, though largely elite, set of backgrounds: nobles, lawyers, entrepreneurs, merchants, bankers, military officers and government functionaries.