He was sent in to Area 51 on a research mission to report back to President Dwight Eisenhower, who warned of an alien invasion.
The claims were made in a video interview in which the 77-year-old man could be clearly seen, but he was not named.
UFO author Richard Dolan interviewed the man who said he was facing kidney failure if an operation was unsuccessful, and may have just months to live.
He claimed to have served in the US Army, worked for the CIA, and worked on the US Air Force Project Blue Book–on studies into UFOs.
His most shocking claim was to have been taken inside Area 51 to see a variety of UFOs allegedly recovered by the US military.
These were said to include a flying saucer said to have crash landed at Roswell, New Mexico, in July 1947.
Area 51 is a heavily guarded US Air Force base deep in the Nevada desert, where tests are carried out on new air craft technology.
But, it is at the centre of many alien conspiracy theories which allege that recovered alien craft that have visited Earth are stored and tested there.
The conspiracy theories were compounded by the fact that the US Government did not acknowledge the base existed until 2013.
Other theories suggest aliens, from the crashed UFOs are secretly kept there as well.
The man says in the video that President Dwight Eisenhower had warned of a threat of Earth being invaded, so he and his superior at the CIA were allowed inside Area 51 to report back to the president.
He claimed the pair were also taken to the alleged S-4 facility southwest of Area 51 where they observed “live extraterrestrials”.
Author and UFO researcher Peter Robbins, said of the account: “In my opinion, if it can be confirmed by any relevant supporting documentation, this moving and fascinating account does qualify as an authentic ‘death bed’ testimony from an individual who convincingly claims to have been a CIA officer deeply involved in the matter or crashed and/or recovered craft, and at one time a liaison between the agency and the President (Truman).”
The interview was filmed in 2013, but only released on the Open Minds UFO website, but is being discussed after being shared on other UFO forums.
It is not clear if the man did die soon after the recording.
It comes after a video interview of a new witness to the Roswell crash emerged, as revealed by Express.co.uk.