She played in Iphigenia in Aulis in Broadway's Circle in the Square Theatre in 1968, and in Medea in 1973. She continued to appear on stage from time to time, including in the Greek Popular Theatre in Athens in 1958, and in New York in Dostoevsky's The Idiot and Jerome Lawrence and Robert E. Papas began her acting career in Greece in variety and traditional theatre, in plays by Ibsen, Shakespeare, and classical Greek tragedy, before moving into film in 1951. She found the acting style advocated by the School old-fashioned, formal, and stylised, and she rebelled against it, causing her to have to repeat a year she eventually graduated in 1948. She was educated from age fifteen at the Royal School of Dramatic Art in Athens, taking classes in dance and singing.
The family moved to Athens when she was seven years old. She recalled that she was always acting as a child, making dolls out of rags and sticks after a touring theatre visited the village performing Greek tragedies with the women tearing their hair, she used to tie a black scarf around her head and perform for the other children. Her mother, Eleni Prevezanou (Ελένη Πρεβεζάνου), was a schoolteacher, and her father, Stavros Lelekos (Σταύρος Λελέκος), taught classical drama at the Sofikós school in Corinth. Papas was born as Irini Lelekou (Ειρήνη Λελέκου) in the village of Chiliomodi, outside Corinth, Greece.