De León studied law at the University of San Carlos and then at the Rafael Landívar University, where he ran the "Sol Bolivariano" ("Bolivarian Sun") newspaper. After graduating he became a state civil servant, working in the Common Market division of the Ministry of Economy between 1967 and 1969, when he became the permanent secretary of the tariff committee. In 1970 he became permanent secretary for the National Committee for Economic and Political integration. During this time he joined the right wing National Liberation Movement (MLN), whose candidate Carlos Arana won the 1970 presidential elections, and during Arana's four-year term de León served as secretary general of the State Advisory Board. He then moved to the private sector, joining the Guatemala Association of Sugar Producers, serving as their legal advisor between 1978 and 1981, and then as their general administrator until 1983. He then became involved in the bloodless coup that saw President Efraín Ríos Montt replaced by Óscar Humberto Mejía.