She was the daughter of Estela Mary Pastore and Jorge Onetto, an employee of the energy company Segba (Servicios Eléctricos del Gran Buenos Aires) and restaurateur who died of a sudden myocardial infarction in 1967 when María was one year old.

She was enrolled in a Catholic school, and at age 17 she began studying psychology at the University of Buenos Aires, where she took up acting in the theater.

She left in 1996, planning to study literature, and moved to Benavídez, Argentina, but found she did want to be an actress, which she began with Rafael Spregelburd's production Dragging the Cross,[a] after which she ceased other work to focus solely on acting.

Power) directed by Norman Briski, which takes inspiration from Noh theater and in which Onetto plays a male girl's kidnapper during Argentina's last dictatorship.

[5] Information released by the Buenos Aires City Police confirmed that she had died by suicide.