His family later moved to Stoughton, Massachusetts, where he worked at his father's filling station business, and he and his brothers became amateur radio operators.
In February 1947, Mackiernan missed the adventure of the war and applied to the State Department for a position as a consular clerk at his former location in China.
He was sent to Peitashan during the Battle of Baitag Bogd on June 19, 1947, to meet with Chinese Hui, Salar, and Kazakh forces, who were fighting both the Outer Mongols and the Soviet Union.
[6] In the CIA, his scientific background (he had dropped out of MIT after his freshman year[7]) were employed in espionage and other intelligence of the Soviet atomic bomb.
[9] In the fall of 1949, Mackiernan led a party of five (including the two men who would survive the trip, Vasili Zvansov and Frank Bessac) out of Ürümqi.
They first spent time with Osman Batur and his Kazakh warriors, who fought against the Chinese Communists, who were invading the Second East Turkestan Republic, and then traveled on to Tibet by horseback and camel en route to India.
With imminent threat of the Chinese invasion, Tibetan guards had standing orders in the tense spring of 1950 to shoot all foreigners who attempted to enter Tibet.
Mackiernan was ordered to stay behind, officially to destroy consular records and equipment and covertly to continue atomic intelligence activities.
On August 10, 1949, Mackiernan sent a classified coded message to Acheson that acknowledged that he was operating the long-range atomic explosion detection equipment.
Two days later, Mackiernan and a Fulbright scholar, Frank Bessac, drove out of the main gates of Ürümqi with their gear, which included machine guns, grenades, radios, gold bullion, navigation equipment, and survival supplies.
Mackiernan and Bessac met up with three anti-communist White Russian allies and rode out to spend more than a month with the Kazakh leader Osman Bator.
Mackiernan left gold and a radio with Osman, who was seen by the invading Chinese as a rebel taking US support and saw himself as a man fighting for the independence of his people.
By late November, the party reached the 10,000 ft "foothills" of the Kunlun Mountains, where it spent the winter with Hussein Taiji of the Kazakhs.
According to Heinrich Harrer, who later befriended Bessac in Lhasa, the Tibetan soldiers who attacked Mackiernan's caravan had hoped to plunder their provisions but were later punished for their callousness.