Most likely the first film to ever use follow-focus. D.W. Griffith convinced his most trusted cameraman, G.W. Bitzer, to fade out the background when the three gangsters walk towards the alley in the opening scene. During this era a cameraman was judged on how sharp and clear his picture was, so Griffith had to take him to an art museum and show him how the background was out of focus and the characters were in focus to convince him to do the effect on the shot. The focusing method is still used.
Widely considered the first modern American gangster film. Gang leader The Snapper Kid - performed by the tragically short-lived actor Elmer Booth - provided a cocky, enterprising prototype for later Hollywood actors (Cagney, Bogart, Robinson, Raft, etc.) who played hoodlums onscreen through the 1940s.
In a street sequence, a young girl can be seen staring at the camera. This was not a mistake; D.W. Griffith had noticed that, in documentary films, people tended to stare at the camera, and felt that having her do just that would make the scene feel more realistic.
In 2016, "The Musketeers of Pig Alley" was selected for preservation by the National Film Registry of the Library of Congress as being "culturally, historically, or aesthetically significant".
This film heavily influenced Martin Scorsese in the making of his own gangster films, Goodfellas (1990) and Gangs of New York (2002). It was picked by Scorsese for his 2005 tribute at Beaubourg, centre d'art et de culture Georges Pompidou (1977) in Paris, France. Biograph is the oldest movie company in America and, as of 2009, is still in existence.