Horn was born in Athens in 1921, the son of Pantelis Horn, a famous dramatist. He studied at the Drama School of the National Theater of Greece, where he made his stage debut in 1941. During his career, he co-operated many times with the National Theater and made personal stage groups with famous actors such as Mary Aroni, Alekos Alexandrakis and Ellie Lambeti. The latter was also his life companion from 1953 to 1958.
He established from an early stage a reputation as "the best actor of his generation," giving outstanding performances in classics such as Gogol's "Diary of a Madman" by Nikolai Gogol, "Richard III" by William Shakespeare, "Dom Juan" by Molière, "Enrico IV" by Luigi Pirandello etc.
Equally important was his screen work; he starred in only ten films, but most of them have attained legendary status, like The Counterfeit Coin (1954), The Girl in Black (1956) and We Have Only One Life (1958). Later he married shipping heiress Anna Goulandri, and became the first director of the Greek State Radio and Television after the restoration of democracy. He died in 1998, but will always be fondly remembered as the jeune premier par excellence of the Greek stage and screen.