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.

During this time, he joined the right-wing National Liberation Movement (MLN), whose candidate, Carlos Arana, won the 1970 presidential elections.

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.

On 25 May 1993, President Jorge Serrano led an autocoup – a coup d'état by someone(s) in the country's government – dissolving the constitution and the National Congress with the apparent support of the army.

On 5 June Espina resigned, and after an overwhelming vote of confidence from the reconvened National Congress, de León was quickly sworn in as President until 14 January 1996, the day Serrano was due to finish his term.

De León promised to defend public freedoms and the rule of law, make progress in the negotiations with the guerrillas, and purge the armed forces of their bad apples.

Both the murder of Supreme Court chief Eduardo Epaminondas González Dubón on 3 April and a massacre of civilians by soldiers in Xamán, department of Alta Verapaz, on 5 October 1995, created high levels of tension in the country, and put strains on the peace process.

On 12 November 1995, a new presidential election was held, and in the second round on 7 January 1996, Álvaro Arzú won to succeed de León.

Then, on 11 March 2002, he resigned his seat in the Congress and from the FRG, declaring that he wished he had never accepted their leader Efraín Ríos Montt's invitation to join the party.