Michael Scanlon, 35, told US District Court Judge Ellen Huvelle that he pleaded guilty to one count of conspiring to bribe public officials in concert with lobbyist Jack Abramoff and defrauded Native American tribes of millions of dollars.
The pair were accused of duping four Native American tribes who had been seeking support from Washington for their interests in gambling enterprises.
One of the officials under a cloud in the Scanlon case is former House of Representatives Majority Whip Tom DeLay, who is also battling separate criminal conspiracy and money laundering charges in Austin, Texas.
Although Mr DeLay has not been charged in the Scanlon-Abramoff probe, his trips and fundraising activities have been closely scrutinised because of his close ties to Mr Abramoff.
In an 18-page plea agreement made public after the hearing in Washington, prosecutors detailed evidence against Scanlon that would have been presented had his case gone to trial, according the Chicago Tribune newspaper.
Scanlon and his lawyers reportedly agreed to all the evidence.
He also agreed to make restitution to the amount of US$19.6 million (A$26.6m), one-half of the amount that prosecutors say he and Abramoff split in profits from the four tribes.
Scanlon has been released on a US$5 million unsecured bond agreement and must give notice to the court and prosecutors before making trips out of the country.
He could face up to five years in prison.