In 1996 at the age of fifteen Greenewald became curious about UFOs and began researching and utilizing the Freedom of Information Act (FOIA) to gain access to thousands of records from the CIA, FBI, Pentagon, NSA, and other groups. Currently, he has accumulated over two million documents on UFOs, the JFK Assassination, nuclear weapons, top secret aircraft, and many others subjects. He began making the documents he obtained freely available online at his website, The Black Valut, which has now become the largest, private, online collection of government documents in the world. Greenewald published his first book, Beyond UFO Secrecy, in 2002. He has regularly writen, produced, and directed for television networks such as The History Channel, Discovery Channel, The Learning Channel, A&E, FOX, NBC, along with international networks such as the BBC (UK) and NTV (Russia). Greenewald started his podcast, The Black Vault Radio, in 2018.

In December of 2017, Greeneewald launched The Black Vault Investigations (TBVI), a volunteer-project to coordinate and perform research on UFO reports. Greenewald said “I have seen many UFO groups come and go, and I have seen many researchers/investigators get frustrated even to the point of leaving the field entirely over the politics and the material being presented. My aim with this project is to change that.” The project includes a public case file database, telephone system for the general public to report cases, and internal system for researches to coordinate their efforts. TBVI is currently seeking individuals interested in taking part. More details can be found here.