Outside the courthouse, Jones broke down in tears as she apologized for her actions, saying she fully understands she has disappointed her friends, family and supporters.
"I have let them down. I have let my country down, and I have let myself down," she said. "It is with a great amount of shame that I stand before you and tell you I have betrayed your trust.
"I recognize that by saying I'm deeply sorry, it might not be enough and sufficient to address the pain and hurt that I've caused you. Therefore, I want to ask for your forgiveness for my actions, and I hope you can find it in your heart to forgive me."
In court, Jones, seated at the defense table and speaking in a clear voice through a microphone, Jones said she lied to a federal investigator in November 2003 when he asked if she had used performance-enhancing drugs.
"I answered that I had not. This was a lie, your honor," she said.
She also pleaded guilty to a second count of lying to investigators about her association with a check-fraud conspiracy.
Jones said she took steroids from September 2000 to July 2001 and said she was told by her then-coach Trevor Graham that she was taking flaxseed oil when it was actually "the clear."
"By November 2003, I realized he was giving me performance-enhancing drugs," she told the judge.
She said she "felt different, trained more intensely" and experienced "faster recovery and better times" while using the substance.
"He told me to put it under my tongue for a few seconds and swallow it," she said. "He told me not to tell anyone."
Jones was released on her own recognizance and was due back in court Jan. 11 for sentencing.
Appearing on the CBS Saturday Early Show with Maggie Rodriguez, WFAN Radio sports talk show host Chris Carlin doesn't buy the flaxseed oil excuse. "It's a little tough to take, to be honest. She's admitting all of this wrongdoing but still not admitting that she knowingly took steroids at the time, it's after the fact.
"Listen, for an athlete not to know what's going in their body - especially an Olympic athlete - at all times, it's a little tough to believe."
It was a stunning fall from grace for Jones, once the most celebrated female athlete in the world. Although she never accomplished her audacious goal of winning five gold medals at the Sydney Olympics, she won three gold and two bronzes.
On the track, the world was awed by Jones' speed, warmed by her smile. Considered at the time the greatest woman sprinter ever, she turned those medals into millions. Advertisers adored her. She was paid 50 to 70 thousand dollars to run in races worldwide.
When allegations first serviced about steroids, she didn't simply deny it: She vehemently denied all doping allegations, even issuing this emphatic declaration in 2004: "I have never, ever used performance-enhancing drugs." She also sued a San Francisco lab owner for defamation after he accused Jones of using performance-enhancing drugs and said he watched her inject herself.
Today, dressed in a dark suit and pink shirt, Jones was somber when she arrived at U.S. District Court with her mother and her attorney, biting her lower lip as reporters and cameras swarmed her. Her mother stumbled at one point but got up and accompanied her daughter inside, where she was fingerprinted and processed before the hearing.
The flaxseed oil Jones said was given to her actually was "the clear" - a performance-enhancing drug linked to the Bay Area Laboratory Co-Operative (BALCO), a San Francisco lab at the center of the steroids scandal in professional sports.
Former sprint record holder and Jones' ex-boyfriend Tim Montgomery as well as recently crowned U.S. Major League Baseball career home run leader Barry Bonds also have been linked to BALCO, and were two of more than two dozen athletes who testified before a grand jury in 2003.
Bonds, like Jones previously, has denied ever knowingly using banned substances.
The International Olympic Committee already has opened an investigation into doping allegations against Jones in December 2004, and said Friday.
Under statute of limitations rules, the IOC and other sports bodies can go back eight years to strip medals and nullify results. In Jones' case, that would include the 2000 Olympics, where she won gold in the 100 meters, 200 meters and 1,600-meter relay and bronze in the long jump and 400-meter relay.
In addition to any jail term, Jones could face a long competition ban from the U.S. Anti-Doping Agency.
"I don't feel any sense of vindication," said BALCO founder Victor Conte, who was sued by Jones for $25 million in 2004. "I feel very sad for Marion and her entire family. I'm sure their pain is great and they are in need of forgiveness.
"All of us have made poor decisions in our lives and suffered the consequences. Marion is not a bad person."
Suspicions and doping allegations have dogged Jones for years. Her ex-husband, C.J. Hunter, was found guilty of doping, and Montgomery, the father of her son Monty, was stripped of his world record in the 100 meters in connection with the BALCO case.
Jones herself was one of the athletes who testified before a grand jury in the BALCO investigation. And in August 2006, one of her urine samples tested positive for EPO, but she was cleared when a backup sample tested negative.
"It cost me a lot of money to defend myself," Conte said Thursday. "But I told the truth then, and I'm telling it now."
"What we see here is sports at its worse moment in history, in which you cannot trust your athletes any more to be clean and playing without juice," Sports Illustrated writer Luis Fernando Llosa told CBS News national correspondent Byron Pitts.
Carlin noted that the prevalence of athletes falling from grace over drug use is rising because of the money involved.
"When you're talking about athletes [like] Marion Jones, you know, she has millions and millions of dollars that she earned from all of this, from her five medals. [It's] a big temptation. You certainly understand it and there's a big difference in a couple of tenths of a second, [it] can be a huge difference money-wise."
Rodriguez noted that while accusations of steroid use have dogged some major athletes like San Francisco Giant Barry Bonds, fans still show up.
"I think the fans mind," Carlin said. "I think the fans have a little bit of a moral issue with it, but at the same time, they still love the game so much that it's hard for them to stay away. Baseball in particular, they're setting all kinds of attendance records. I don't think it's about hitting home runs or anything like that - it's about the game.
"Barry Bonds broke the most important record in all of professional sports and nobody really cares about it," Carlin noted. "I think that should tell you everything you need to know."
Carlin said harsher punishment, such as jail time (Jones faces a minimum of six months in prison) is warranted. "The more harshly they get punished, the better it will turn out but at the same time, as long as there are steroids and performance-enhancing drugs out there that there is no test for the moment they're going to keep doing it because again it's more money in their pocket and that ultimately and sadly is what it's all about.
The saddest part of Jones' tragic tale is that athletes with as much natural talent as she has certainly doesn't need such drugs to perform. "You can understand almost to a degree some of the guys who don't quite have as much talent and are trying to get over the top, but the superstars? Barry Bonds was going to be a Hall of Famer anyway and he goes on and brings himself nothing but a whole lot of aggravation, and certain no respect from the fans whatsoever."