A career Army man, de Llano was a brigadier general in 1923 when he began to speak out against the army and Miguel Primo de Rivera. Demoted, he served three years in prison but refused to stop criticizing even on his release, as a result of which he was dismissed altogether in 1928. In 1930, he became a revolutionary but on a failed attempt to overthrow King Alfonso XIII, he fled to Portugal.
He returned to his native land in 1931 after the departure of Alfonso XIII and assumed command of the 1st Military District of the Spanish Republican Army.
Even as he rose in prominence, he remained critical of the shifting governments, joining in on a plot to overthrow the Popular Front government in May, 1936. During the Spanish Civil War, de Llano secured the capture of Seville with a force of at least 4,000 troops.
There, he ordered mass killings. Subsequently, he was appointed the commander of the Nationalist Army of the South.