He attended Princeton University, graduating with a degree in archaeology and also summa cum laude.
Bagby's focus was on police investigators, especially the fictional Inspector Schmidt, Chief of Homicide for the New York Police Department. In the Schmidt novels, mystery-writer Bagby himself appears as "the Watson to Schmidt's Holmes, following him on cases, and acting as biographer.". A number of his novels have been translated into other languages, including German, French, and Spanish.
His early avante-garde novels came to the attention of Theodore Dreiser and were published, but he did not gain much fame till he moved into writing mysteries.
In addition to Bagby, he also published mystery novels under his own name, and under the pseudonym Hampton Stone.He held a position as a radio critic for a New York newspaper in the 1930s, and then went to work for Time magazine. During World World War II he worked with the United States Army.
His final book, The Garbage Collector, was published in 1984. In addition to the hero of most of the Bagby novels, Inspector Schmidt, Stein also created a New York City Assistant District Attorney named Jeremiah Gibson for the books published under the Stone pseudonym, and archaeologist detectives Tim Mulligan and Elsie Mae Hunt, as well as engineer Matt Herridge, for the mysteries published under his own name. Stein's first novel was published in 1930.
His first mystery, Murder at the Piano, was published in 1935.
The first novel written as Stone was titled The Corpse in the Corner Saloon. lieutenant was reviewed in the New York Times in 1948.
With United States Army, 1943-1945. Member Mystery Writers American (president 1974, Grand Master Edgar award, 1979), Princeton Club (New York ), Nassau Club, Phi Beta Kappa.