Accurate accounting of Poshepny's career is complicated by government secrecy and by his tendency to embellish stories. For example, he often claimed to be a refugee from Hungary, but was actually born in Long Beach, California. He joined the US Marine Corps in 1942 and fought in Iwo Jima, receiving two Purple Hearts. In 1951 he joined the CIA and worked in Korea during the Korean War, training refugees for sabotage missions behind enemy lines.
After the Korean war, Poshepny joined the Bangkok-based CIA front company Overseas Southeast Asia Supply (SEA Supply), which provided military equipment to Kuomintang forces based in Burma. In 1958 Poshepny tried unsuccessfully to arrange a military uprising against Sukarno, the president of Indonesia. From 1958 to 1960 he trained various special missions teams, including Tibetan Khambas and Hui Muslims, for operations in China against the Communist government. Poshepny sometimes claimed that he personally escorted the 14th Dalai Lama out of Tibet, but sources in the Tibetan exile deny this.
The agency was impressed with Poshepny's ability to train paramilitary forces quickly and awarded him the CIA Star in 1959. Two years later he was assigned to train Hmong hill tribes in Laos to fight North Vietnamese and Pathet Lao forces. He and his CIA counterpart in Laos, J. Vinton Lawrence (aka James Vinton), were closely associated with both the Hmong leader, General Vang Pao, and the CIA-owned Air America. In Laos, Poshepny adopted practices that were barbaric even by agency standards. He paid fighters to bring him the ears of dead enemy soldiers, and on at least one occasion he mailed a bag of ears to the US embassy in Vientiane to prove his body counts. He dropped severed heads onto enemy locations twice in a grisly form of psy-ops. Violating agency orders, he fought alongside the native forces. Poshepny's courage and ruthlessness earned him the respect and loyalty of the Hmong, but they shocked his superiors in the agency. Seriously wounded by shrapnel three times, he started drinking heavily. The CIA extracted him in 1970 and, despite his violations, awarded him another CIA Star in 1975.
Poshepny led a quiet retirement, living in California with his Hmong wife and three children. He helped veterans immigrate and settle in the US and was a frequent guest at veteran gatherings. He freely admitted his actions during the war to reporters and historians, saying they were a necessary response to communist aggression.
Several press stories have suggested that Poshepny was the model for Col. Walter Kurtz in the film Apocalypse Now, but both Poshepny and director Francis Ford Coppola have denied the connection.