Hamilton admitted taking an herbal product for two days in February to combat depression, knowing it included a steroid.
"There's nothing to fight about," the 38-year-old Hamilton told The Associated Press. "I took a banned substance. I accept the consequences. You make mistakes in your life and I accept the penalty like a man."
Hamilton will likely receive a ban from the U.S. Anti-Doping Agency that might range from eight years to life, a sentence that would have ended his racing days anyway.
"He has had a cloud over his career for a while now and the sport is better off without him," said Pat McQuaid, president of the International Cycling Union.
Hamilton's win at the 2004 Athens Games was overshadowed by a blood doping scandal. He tested positive for doping a second time later that year, served a two-year suspension and returned to racing early in 2007 -- never revealing that he was fighting depression, which he said runs in his family.
Going through a divorce and seeing his mother fight breast cancer made things worse in recent months, Hamilton said.
Seeking relief, he took something called Mitamins Advanced Formula, billed as a "natural depression treatment with vitamins, herbs and supplements."
"Obviously, that was a mistake," Hamilton said.
In a statement, USADA said it will continue going through the process of issuing a sanction.
The Mitamins product contains common things such as vitamins D, B-6 and B-12, along with thiamin, riboflavin and calcium. Each serving also contains 20 milligrams of a steroid called Dehydroepiandrosterone -- DHEA, as it's known.
"I took it to help my mental state," Hamilton said. "I did not, 100 percent, take it for any performance enhancement."