After years of denials, the 41-year-old Texan told chat show host Oprah Winfrey that he had used the blood-boosting agent EPO, as well as taking testosterone, human growth hormone, cortisone and blood doping.
In the much-anticipated interview Armstrong, who last October was stripped of all his Tour titles, said that at the time of his drug-taking he did not feel it was wrong.
He said he did not feel bad about taking performance-enhancing drugs, nor did he feel it was cheating, as he was creating a level playing field with other riders who took drugs.
He said: "I looked up the definition of a cheat: to gain an advantage. I didn't view it that way. I viewed it as a level playing field."
But he said he had now changed his opinion, telling her: "I'll spend the rest of my life trying to earn back trust and trying to apologise to people. For the rest of my life.
"I see the anger in people. And betrayal. It's all there. These are people that supported me, believed in me. They have every right to feel betrayed. And it's my fault.
"I will spend the rest of my life trying to earn back trust and apologise to people.
"I made my decisions. They are my mistake. I am sitting here today to acknowledge that and to say I'm sorry for that. I deserve this."
He added: "I'm happier today than I was then."
Armstrong told Winfrey he felt doping was necessary to win the Tour de France.
He said: "That's like saying we have to have air in our tyres or water in our bottles. It was part of the job.
"I don't want to make any excuses, but that was my view and I made those decisions."
Armstrong, who has been stripped of all his results from August 1, 1998 and banned from sport for life, denied doping during his comeback from retirement in 2009 and 2010.
And he said he wished he had co-operated with the United States Anti-Doping Agency (WADA) investigation, which proved his downfall.
He said: "This story was so perfect for so long," Armstrong, who confirmed his doping in a series of answers to yes-no questions, told Winfrey.
"I try to take myself out of this situation and look at it: you overcome the disease, you win the Tour de France seven times, you have a happy marriage, you have children.
"It's this mythic, perfect story and it wasn't true."
The myth of the cancer survivor turned serial winner, which Armstrong perpetuated, captivated millions.
Asked if it was hard to live up to that image, Armstrong said: "Impossible. The story is so bad and so toxic, a lot of it's true."
In the interview, recorded on Monday in his home city of Austin, Texas, Armstrong was asked why he chose to confess his misdemeanours now.
"I don't know that I have a great answer," he said.
"This is too late. It's too late for probably most people and that's my fault.
"I view this situation as one big lie, that I repeated a lot of times. It wasn't as if I just said no and I moved on."
Armstrong insisted it was his successful battle with testicular cancer, chronicled in his books and which prompted the establishment of the Livestrong charity, which increased his desire to win at all costs.
His decisions were based on a "ruthless desire to win" and, he said, did not feel wrong to him at the time.
During an era where doping was prominent, Armstrong viewed his drug-taking, which began before his cancer diagnosis, as "a level playing field", rather than cheating.
Armstrong was able to bypass the testers by clever "scheduling".
He felt, due to his successful battle with cancer, he could excuse his use of testosterone.
He said: "My cocktail, so to speak, was only EPO, but not a lot, transfusions and testosterone, which in a weird way I almost justified because of my history, with having testicular cancer and losing (a testicle); 'Surely I'm running low'.
"There's no true justification for those (blood transfusions)."
Until now, Armstrong was vehement in his repeated denials and fought any allegations made, winning numerous lawsuits; his admission now opens him up to possible counter suits.
Armstrong's defence long relied on the fact he had never tested positive - a stance he maintains.
"I didn't fail a test," he said. "Stuff was retroactively tested. Technically, retroactively, I failed those. The hundreds of tests I took, I passed them."
Armstrong insists he is willing to co-operate with the authorities in future.
"If there was a truth and reconciliation commission, if they had it and I was invited, I'd be the first man through the door," he said.
The second part of the interview will be broadcast tomorrow.