The seven-times Tour de France winner stepped down as the foundation's chairman on Wednesday after a doping scandal badly tarnished his cycling career.
But Armstrong still took the stage at a gala dinner in Austin, Texas, on Friday to celebrate the 15th anniversary of the charity's founding.
"This mission is bigger than me - it's bigger than any individual," Armstrong said of the foundation, which helps people and families affected by cancer.
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"We will not be deterred; we will move forward," he said.
Armstrong, who is expected to learn on Monday whether he will be stripped of his titles, said that when people ask how he is, he says: "I've been better but I've also been worse."
And he then sought to put his professional troubles to one side, telling the 1,500 gala guests: "Let's have a hell of a good time tonight."
The 41-year-old Armstrong founded Livestrong in 1997, after he was diagnosed with testicular cancer and before he first won the Tour de France. Since then, it has raised nearly $500 million and has evolved from a focus on testicular cancer research to addressing the needs of survivors of all cancers.
Armstrong, who lives in Austin, is still on Livestrong's board and despite the doping scandal, the $1,000-a-head gala, which featured musicians Norah Jones and Stephen Marley and actors Sean Penn, Matthew McConaughey and Robin Williams, is expected to raise $2.5 million.
Guests, some wearing dresses or ties in the foundation's signature yellow, bid on auction packages, including a dinner with Armstrong that required a starting bid of $3,000.
So far the foundation's financial health appears not to have suffered from the scandal and contributions have actually risen this year as the probe gathered momentum. For the year 2012 so far, it has reported revenue of $33.8 million, up 2.1 percent from the same period of 2011.
Armstrong is expected to lose his record seven Tour de France titles after the U.S. Anti-Doping Agency published a 1,000-page report last week that said Armstrong organized and took part in an elaborate, sophisticated doping scheme on his way to his unrivalled success.
The retired cyclist has always denied he took banned substances during his career but decided not to challenge the USADA charges against him.
Cycling's world governing body, the International Cycling Union, is expected to rule on Monday on the USADA report. It can either confirm Armstrong's life ban and strip him of his Tour titles or take the matter to the Court of Arbitration for Sport.
Sportswear maker Nike Inc and beer maker Anheuser-Busch were among corporate sponsors who said this week that they would end their relationship with Armstrong, but continue to back the foundation.
Feelings were mixed at Friday's gala.
Jeff Bennett of Portland, Maine, who was diagnosed with breast cancer nine years ago, said just before he walked down the yellow carpet into the Austin Convention Center that he came to support both Livestrong and Armstrong.
"Livestrong has provided a ton of opportunities for me to give back," he said. "Lance started it."
Michael Parmet, a cycling enthusiast from the Houston area, had a slightly different view, saying: "We believe in the purpose, not necessarily the man."
However, he added: "He's still the greatest athlete in the dope generation. He didn't do anything anyone else didn't do."
The doping scandal that has engulfed Armstrong extended further into the cycling world on Friday, when Dutch bank Rabobank ended its multi-million euro backing of professional cycling.
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