The 40-year-old, who has never tested positive for a banned substance, decided not to contest charges against him, accusing the cyclist of taking performance-enhancing drugs since 1996.
The USADA was expecting to strip Armstrong, who survived testicular cancer, of his infamous seven titles in the Tour de France as well as prevent the American from competitively cycling for life.
A statement from the agency said: "The evidence against Lance Armstrong arose from disclosures made to USADA by more than a dozen witnesses who agreed to testify and provide evidence about their first-hand experience and/or knowledge of the doping activity of those involved in the USPS conspiracy as well as analytical data.
"As part of the investigation Mr Armstrong was invited to meet with USADA and be truthful about his time on the USPS team but he refused. "
Armstrong recently claimed that he wishes to spend time with his family and continue his charity work instead of fighting the doping charges.