Cecilia Sophia Anna María Kalogeropoúlou was born on 2 December 1923 in New York. The Kalogeropoúlou family moved from Greece to America only a few months before María's birth. To become more "American", her father changed their family name to Callas. During 1937, María moved back to Greece with her mother, and again changed her family name back to Kalogeropoúlou. She then became a student of the well-known singer Elvira de Hidalgo.
She made her stage debut on 2 April 1939 in Athens, in a performance of Mascgani's Cavalleria Rusticana - when she sang the female lead role of "Santuzza" - at the age of exactly 15 years and 4 months.
Before this debut, she sang in a few amateur and school concerts/productions, and also at least one radio broadcast.
The first time she ever sang in public was at the end of 1934, when she sang "La Paloma" and "A Heart That's Free" in a radio contest (nationwide amateur hour for children) in New York - just 11 years old.
Other activities included small roles in Gilbert & Sullivan operas, and a few arias/songs in concerts.
A recording exists of a radio broadcast on 7 April 1935, where "Un bel di vedremo" (from Madama Butterfly) is sung by Nina Foresti, and some believe this to be the 11-year-old María Kalogeropoúlou. However, it seems highly unlikely that it is the case, since the address and the handwriting of this young singer do not match that of the young María, and it is almost certain that she only studied the aria for the first time during the early 1950's.
In the years 1939-47, the following were highlights of her early career, when she sang almost exclusively in Athens:
She made her stage debut in a leading role on 2 April 1939, as "Santuzza" in Cavaleria Rusticana (Mascagni)
1939: Sang the role of "Santuzza" in Cavalleria Rusticana (Mascagni) 2 times.
1940: Sang the role of "Suor Angelica" (Puccini) once.
She made her professional debut on 15 February 1941, as "Beatrice" in Boccaccio (Suppé)
1941: Sang the role of "Beatrice" in Boccaccio (Suppé) several times.
Her first notable success came on 27 August 1942, as Tosca (Puccini)
1942: Sang the role of "Tosca" (Puccini) approximately 9 times - in Greek as well as in Italian.
1943: Sang the role of "Tosca" (Puccini) 3 times - in Italian.
1944: Sang the role of "Marta" in Tiefland (d'Albert) 8 times, the role of "Smarágda" in O Protomástoras (Kalomíris) 6 times, the role of "Santuzza" in Cavalleria Rusticana (Mascagni) 2 times, and the role of "Leonore" in Fidelio (Beethoven) 11 times.
1945: Sang the role of "Marta" in Tiefland (d'Albert) 3 times, and the role of "Laura" in Der Bettelstudent (Millöcker) 7 times. Towards the end of 1945, María Kalogeropoúlou went to New York, where she sang in several auditions, and studied. She did not appear on stage again before her major breakthrough in Verona on 2 August 1947, when Maria Callas sang the role of "Gioconda" (Ponchielli).