Johnson, who was banned from athletics for using steroids at the 1988 Olympics, told BBC Radio Five Live: "American people will forgive him. I think people will judge him differently, based on what he did for humanity and for cancer.
"I don't think it will be tough for him to make a living. I hope he can move on and do good things. If he can find some way to make a living he will be fine."
Armstrong confirmed that he had used EPO, testosterone, cortisone and blood transfusions during his career.