Edward Caraballo, a television cameraman from New York, was flown out of Afghanistan under heavy US security on Sunday.
Caraballo said he was overjoyed to be freed and would return to the US to make a film documenting "the truth" about the secret Kabul prison.
Afghan authorities discovered the jail, which contained eight Afghans who complained of being tortured, in July 2004.
The prison was run by former US soldier Jonathan "Jack" Idema and another US citizen, Brent Bennett – who were also imprisoned on charges of torture and running a private jail.
The three men say the US government sanctioned the jail which Idema said was part of a freelance hunt for "terror suspects".
The US military acknowledges accepting a prisoner from Idema, but insists it realised shortly afterwards that Idema was an impostor.
The Afghan authorities were enraged by the discovery, which took place around the same time as the Abu Ghraib prison scandal occurred in Iraq.