He has worked on updating the Roman Law book named "Lecciones de Derecho Romano", which is now on its seventeenth edition.
Medellín was appointed Minister of Justice and Law of Colombia by President Ernesto Samper Pizano on 11 January 1996, replacing Néstor Humberto Martínez Neira.
The law implied that in case there was a condemnatory sentence, the government would become the owner of the things acquired by drug money or in an illegal way.
[4][5] On 7 December 2006, Medellín was appointed Ambassador of Colombia to the United Kingdom of Great Britain and Northern Ireland, and Concurrent Non-Resident Ambassador to the Republic of Ireland, and presented his credentials to Queen Elizabeth II in a ceremony at Buckingham Palace on 2 March 2007.
His nomination for Ambassador to The Hague was a matter of great importance, given the case at the Permanent Court of Arbitration on the sovereignty claims of Colombia over the Archipelago of San Andrés, Providencia and Santa Catalina after Nicaragua presented a dispute case over the territory, and the President believed Medellín's law background would help Colombia's cause in court.
But suddenly, latter that month, Medellín turned down both Ambassadorships, when an article in El Espectador revealed that Carlos Alberto Gaviria Vélez, older brother of José Obdulio Gaviria Vélez (cousin of Pablo Escobar), who was an Advisor to President Uribe, was directly involved with Luis Carlos Molina Yepes, the moneyman involved in planning the assassination of Guillermo Cano Isaza, Medellín's father-in-law.
[7] His resignation was coupled with an earlier one from the Anticorruption Czar, Rodrigo Lara Restrepo, who resigned because José Obdulio Gaviria Vélez had deliberately lied to him about an article published by the Miami based El Nuevo Herald, where they discussed the death of his father, Rodrigo Lara Bonilla.