[3] When he began looking for work in Hollywood, he used a cover letter that began "Dear Sir, I like words" and concluded,I like the word screenwriter better than copywriter, so I decided to quit my job in a New York advertising agency and try my luck in Hollywood, but before taking the plunge I went to Europe for a year of study, contemplation and horsing around.
Pirosh began his film career in 1934 as a junior writer for Metro-Goldwyn-Mayer, working with fellow newcomer George Seaton.
He and Delmer Daves adapted Ayn Rand's Night of January 16th for a 1941 film of the same name directed by William Clemens.
He earned an Academy Award for Best Writing, Story and Screenplay in 1949 for his script for the World War II drama Battleground, a film he also produced, that was the first based on the Ardennes battle.
He would go on to write the story for the highly regarded Steve McQueen World War II film Hell Is for Heroes, directed by Don Siegel, believed[citation needed] to be the basis for TV's Combat!