"For 15 years I hid a lie," Kjaergaard told a news conference. "I had believed it was best for me and the sport to carry this dark secret to the grave."
Kjaergaard, who raced in the 2000 and 2001 Tours, said the U.S. Postal Service team took care of his doping needs but kept information in a "closed circuit".
"When I was a part of the U.S Postal Service team, everything was organised by the team. I did not need to arrange for a doctor or do anything by myself," Kjaergaard said.
"I cannot say if any of my team mates were using illegal substances," Kjaergaard added. "I can assume that others at U.S Postal were using something that the witness reports said. I have no direct knowledge though."
Armstrong was stripped of his seven Tour de France titles and banned for life on Monday after the International Cycling Union (UCI) ratified the United States Anti-Doping Agency's (USADA) sanctions against the American for taking drugs.
"The reason that I am coming forth now is that I have had a big problem with my own conscience," Kjaergaard said.
The Norwegian began using banned substances - primarily erythropoietin (EPO) and cortisone - in 1998, before joining the U.S. Postal Service team, he said.
"During this period there was a new EPO test and we had to resort to intravenous micro-dosages to shorten the window where we could be caught," he said. "I also used other illegal substances on the list but not many."
Armstrong, who denies wrongdoing, lost his 1999-2005 titles after former team mates testified against him and themselves, describing what USADA called the "most sophisticated, professionalized and successful doping program that sport has ever seen".
Harald Tiedemann Hansen, president of the Norwegian Cycling Federation, said it was a "dark day" for the Nordic nation's cycling community and the governing body would conduct an investigation.
Kjaergaard, who was placed on indefinite leave by the federation, said he would eventually leave cycling and pursue a career outside of sport.