Who was the first film maker to win an Oscar for his directorial debut film?

In 1955, first time feature film director Delbert Mann and screenwriter Paddy Chayefsky made the film adaptation of the play "Marty" (which was a two year collaboration of them). Their film won the Academy Award for Best Picture and Best Actor (Ernest Borgnine). It also earned other Oscars, winning for Chayefsky’s screenplay and Mann’s film direction. At the time of winning this award, it caused Mann to be the first film maker to win an Oscar for his directorial debut of a motion picture.

In the film, "Marty" depicts thirty-six hours in the life of the main character: 34 year old, bug eyed Marty Piletti (Ernest Borgnine). He is an ordinary, burly, heavy-set Bronx butcher. In the opening scene in his butcher shop while Marty waits on a female customer, he tells her how all his younger brothers and sisters are happily married and raising families. As a romantic loser all his life, Marty is resigned to listening to people ask him about the point in time when is he going to get married.

Marty in the film finally awakens to his own reality, after thirty-four years of loneliness. He abandons his friends and family and from a phone booth, he calls the person that he wants in the rest of his life as his wife, Clara (Betsy Blair).