He entered the Cortes in 1854 as a Progressist deputy for Zamora.
After the coup d'Nat of Don Leopold O'Donnell in 1856, Sagasta had to go into exile in France, but promptly returned, to become the manager of the Progressist paper La Iberia, and to sit in the Cortes from 1859 to 1863.
He seconded the Progressist and revolutionary campaign of Prim and the Progressists against the throne of Queen Isabella, conspiring and going into exile with them.
He retired to his house in Logrono, which he left six years later, in 1854, when called upon by the queen to take the lead of the powerful Liberal and Progressist movement which prevailed for two years.
The growing ambition of General O'Donnell constantly clashed with the views of Espartero, until the latter, in sheer disgust, resigned his premiership and left for Logrono, after warning the queen that a conflict was imminent between O'Donnell and the Cortes, backed by the Progressist militia.
At the age of seven he lost his father, who had taken an active part in the progressist agitations during the reign of Ferdinand VII., and had passed several years as an exile in England.
From the first he took an important place in the chamber, as one of the most notable orators of the Progressist Republican group. In January 1896 he was elected vice-president of the chamber, and henceforth devoted himself to the struggle against the Left, not only in parliament, but also in public meetings throughout France.
His addresses at Marseilles on the 26th of October 1896, at Carmaux on the 27th of December 1896, and at Roubaix on the 10th of April 1897, were triumphs of clear and eloquent exposition of the political and social aims of the Progressist party.