Prosecutors alleged that Green, then 16, was paid $25 by drug dealer Derrick House to knock on a rival drug dealer's door in 1985. Once inside, House allegedly stabbed, shot and burned four people to death.
Green appealed his conviction, saying he was not at the crime scene. A Cook County judge ruled in August 2007 that Green was entitled to a new sentencing hearing, though not a new trial. He left the Stateville Correctional Facility on Friday, said Illinois Department of Corrections spokeswoman Januari Smith.
House received the death penalty but was released in 2006 after the Illinois Supreme Court reduced his sentence.