In a statement on Livestrong's website, Armstrong, who overcame life-threatening testicular cancer, said he would "conclude my chairmanship ... to spare the foundation any negative effects as a result of controversy surrounding my cycling career."
Separately, Nike, a major sponsor that had stuck by Armstrong in the face of doping allegations, issued a statement that accused him of years of deception.
"Due to the seemingly insurmountable evidence that Lance Armstrong participated in doping and misled Nike for more than a decade, it is with great sadness that we have terminated our contract with him," it said.
Nike would continue support of the Livestrong initiatives, it said. Livestrong is one of the best-known cancer charities in the US, having raised nearly $500 million ( Rs. 2,650 crore) since it was founded by Armstrong in 1997 as he recovered from testicular cancer.
Its iconic yellow wristband was launched in 2004 in collaboration with Nike.
Armstrong always maintained he didn't use banned substances but last week, US Anti-doping Agency alleged that he was at the heart of what it called the biggest doping conspiracy in sports history.