Robertson was the third of five children born to John Chelsea and Mildred Robertson of Birmingham. He dropped out of school at age 17 and took the GED in order to enlist in the U.S. Army in 1954. He and his wife, Wanda, had two daughters.
Robertson joined the special forces and attained the rank of Sergeant First Class in the Army's Green Berets, eventually becoming a member of the Military Assistance Command, Vietnam – Studies and Observations Group (MACV-SOG) during his service in the Vietnam War. On May 20, 1968 he was participating in a classified resupply mission flying into Laos on a Vietnamese Sikorsky H-34 "Kingbee" helicopter crewed by Vietnamese soldiers. The aircraft was shot down by Communist forces in the A Shau Valley, 4 miles inside Laos. Witnesses reported that the craft was engulfed in flames and there was no indication of survivors.
Robertson was promoted to Master Sergeant after the accident, in which he was reported missing in action. Robertson was officially declared dead on April 28, 1976. His name appears on the Vietnam Veterans Memorial in Washington D.C.
According to claims explored in a 2013 documentary film, Robertson was captured by the North Vietnamese and held prisoner in a bamboo cage. He was tortured and accused of spying for four years. After his escape, he married a Vietnamese nurse who had found and cared for him and took the name of her dead husband, Dang Tan Ngoc, eventually raising a family with her. It is further claimed that, due to his injuries and isolation, Robertson lost the ability to speak English and forgot most of his earlier life.
The United States Department of Defense's Defense Prisoner of War/Missing Personnel Office (DPMO) has evaluated several claims by people living in Southeast Asia claiming to be John Robertson since 2002. None of those claims, including those made by or on behalf of Dang Tan Ngoc, have been found credible. U. S. personnel interviewed Dang in Ho Chi Minh City in 2006 and recorded his admission to being a Vietnamese citizen. Dang was fingerprinted during another interview in Phnom Penh in 2008 and his prints did not match Robertson's records.
Veteran Tom Faunce learned about Robertson during a humanitarian mission in 2008. He worked with Canadian filmmaker Michael Jorgensen to record Robertson's story, including a 2012 reunion with Robertson's surviving sister, Jean. The documentary, Unclaimed, premiered in Toronto on April 30, 2013.
- Millner, Rod (February 19, 2009) "History of Reporting Related John Hartley Robertson (Refno 1184)" Defense Prisoner of War/Missing Personnel Office
- Barnard, Linda (April 25, 2013) "Hot Docs premiere Unclaimed finds a Vietnam veteran left behind for 44 years." Toronto Star
- Johnson, Robert (May 1, 2013) "The Man Who Claims To Be Lost US Vietnam Vet John Hartley Robertson 'Is A Fraud'." Business Insider
- Unclaimed film website