In the two-part interview with Oprah [ Images ] Winfrey streamed live online to a global audience, the cancer survivor confessed to using performance-enhancing drugs during a career in which he won the Tour de France seven times.
Armstrong admitted to taking EPO, blood transfusions and other banned substances.
He said doping in team was "professional and smart", but denied taking banned drugs since his comeback in 2005.
Armstrong's world began to crumble in October last year when the US Anti-Doping Agency released a detailed report with sworn testimony from dozens of people that described him as the ringmaster of the "most sophisticated, professionalized and successful doping program that sport has ever seen."
The American was stripped of his seven Tour de France titles and banned for life by the International Cycling Union (UCI) in October after riders testified that he took drugs.
The testimony came in a United States Anti-Doping Agency (USADA) report in which the 41-year-old's former US Postal team was accused of running "the most sophisticated, professionalized and successful doping program that sport has ever seen".