Rosas came on to the national scene as a powerful cattle rancher. He controlled the cattle frontier pampa of Argentina, allowing him to rule over the capital, Buenos Aires. He married right before age 20 on March 16, 1813 to almost 18 year old María de la Encarnación de Ezcurra y Arguibel, born in Buenos Aires on March 25, 1795, daughter of Juan Ignacio de Ezcurra y Ayerra and wife Teodora Josefa de Arguibel y López Cossio. Their only child, daughter Manuela Robustiana de Rosas y Ezcurra, born in Buenos Aires on May 24, 1817 and died in 1898, married Maximo Terrero y Muñoz, by whom she had two sons. He also had an illegitimate son, Mariano Antonio Severo de Balcarce, married in Paris on December 13, 1832 to María de las Mercedes Tomasa de San Martín y Escalada, daughter of José de San Martín. Rosas defeated the European expeditionary forces in 1820 with the help of gauchos.
In December 1829, Rosas became governor of Buenos Aires. In subsequent years, Rosas went in and out of power, but remained a strong leader. During his years out of office (1832-1835), Rosas waged a military campaign against the indigenous population in southern Argentina. His wife Encarnación died in Buenos Aires on October 20, 1838.
As a leader, Rosas portrayed himself as a man of the people, who could relate to the working class of gauchos and Afro-Argentines. Rosas used his man of the people ideal to unify Argentina during his era. Rosas supporters called themselves Rosistas. Rosas rule was filled with violence — he killed his opponents and anyone else who would not support him. To that end, he developed a paramilitary force known as La Mazorca ("the Corncob"), which, by coincidence or by design, was a Spanish homophone for más horca ("more hanging").
The cruelty of his regime and his alignment with the Roman Catholic Church were highlighted by the execution of Camila O'Gorman and the Jesuit priest Ladislao Gutiérrez. O'Gorman and Gutiérrez had fled Buenos Aires in order to continue their love affair and remained in hiding near the Brazilian border. Eventually found and arrested, Rosas decreed their sentence. They were executed before a firing squad on 18 August 1848 in Santos Lugares, near Buenos Aires. Camila O'Gorman was twenty years old and eight months pregnant. Their story was the subject of María Luisa Bemberg's 1984 film Camila.
As Charles Darwin related in The Voyage of the Beagle, he met Rosas, who was then engaged in exterminating tribes of wandering horse-mounted Indians, describing him as a man of extraordinary character, a perfect horseman who conformed to the dress and habits of the Gauchos and "obtained an unbounded popularity in the Camp, and in consequence a despotic power". Darwin included a story of how Rosas had himself put in the stocks for inadvertently breaking his own rule of not wearing knives on Sundays. This appealed to his men's sense of egalitarianism and justice.
Rosas attempted to reincorporate Uruguay and Paraguay as Argentine provinces and this led to two European blockades of Buenos Aires. In 1831, during the early part of Rosas's rule, Luis Vernet, the Argentine Governor of the Falkland Islands, seized United States seal hunting ships for illegal sealing and, in response, a U.S. warship destroyed the Argentine settlement, leaving escaped convicts and pirates on the islands. In November 1832, a new governor arrived in the islands, but was killed in a mutiny, and though a ship's commander took charge, in January 1833, the United Kingdom reasserted its claim to the Falklands and took control. Rosas wanted to rid Argentina of European influence and cultivate a feeling of nationalism among Argentinians.
Rosas' opponents during his rule were dissidents, such as José María Paz, Salvador M. del Carril, Juan Bautista Alberdi, Esteban Echeverria, Bartolomé Mitre and Domingo Faustino Sarmiento. Rosas political opponents were exiled to other countries, such as Uruguay and Chile.
On February 3, 1852, Rosas was overthrown by Justo José de Urquiza, who was supported by Uruguay and Brazil--his army was defeated at Caseros. Rosas spent the rest of his life in exile, in the United Kingdom, as a farmer in Southampton. He was buried in the cemetery on Southampton Common.