Mario O'Donnell (born in 1941), best known as Pacho O'Donnell, is an Argentine writer, politician and physician who specializes in psychoanalysis.
He was born in Buenos Aires to Mario Antonio O'Donnell and Susana Lucrecia Ure, and is married to Susana Evans Civit, with whom he has five children. His brother Guillermo was a noted political scientist.
After the return to democracy in Argentina in 1983, he was named Secretary of Culture of the city of Buenos Aires. He ran unsuccessfully for the 1988 presidential nomination in the Radical Civic Union primaries, but later became a Peronist and was elected to the Argentine Senate for the city of Buenos Aires. President Carlos Menem appointed O'Donnell Ambassador to Bolivia and Paraguay, as well as Secretary of Culture. He was elected to the Argentine Chamber of Deputies in 2001.
He is currently engaged on the diffusion of Argentine historical knowledge, being part of the "neorevisionist" school which contests the official reading of history imposed in Argentina (democracy being restored only in 1983), and wrote Historia confidencial (Confidential History) with fellows historians José Ignacio García Hamilton and Felipe Pigna.
He received the "Isabel la Católica" ("Isabella the Catholic") order from the Spanish King Juan Carlos I of Spain, and the "Palmas Académicas" ("Academic palms") in France. The legislature of Buenos Aires honoured him as illustrious citizen in October 2009.