Shanley was born in The Bronx, New York City, to a telephone operator mother and a meat-packer father. Per his Editorial Review by Amazon, he was thrown out of St. Helene's kindergarten, banned from St. Anthony's hot lunch program and expelled from Cardinal Spellman High School. When asked why he had been treated in this way, he burst into tears and said he had no idea. Then he went into the United States Marines.
In 2012, Shanley wrote the libretto for an opera version of Doubt which premiered at the Minnesota Opera in January 2013, with music by Douglas J. Cuomo. He related how the opera version came about, even though he knew little about opera, in a January 17, 2013 article in the Wall Street Journal.