Horn was born in Athens in 1921, the son of playwright Pantelis Horn (himself descended from an Austrian father and Greek mother), and Euterpi, a Greek. He studied at the Drama at the National Theatre of Greece Drama School, where he made his stage debut in 1941. During his career, he co-operated many times with the Greek National Theater and made personal stage troops with actors such as Mary Aroni, Alekos Alexandrakis and Ellie Lambeti. The latter was also his companion from 1953 to 1958.
From an early stage he developed a reputation as "the best actor of his generation," performing many classics such as "Diary of a Madman" by Nikolai Gogol, "Richard III" by William Shakespeare, "Dom Juan" by Molière, "Enrico IV" by Luigi Pirandello to critical acclaim.
Of lesser importance to him was his screen work; as he disliked cinema only starring in ten films. His most notable films were The Counterfeit Coin (1954), and A Girl in Black (1956). He later married shipping heiress Anna Goulandri, and became the first director of the Greek State Radio and Television after the restoration of democracy. He died of cancer on 16 January 1998.