Prosecutors revealed January's guilty plea by Ruben "Doc" Cavazos in court papers filed on June 29. He could face up to 20 years in prison when he is sentenced in February.
A phone message left with Cavazos' attorney Angel Navarro was not immediately returned Tuesday.
Cavazos and other Mongol leaders directed members to engage in murders, extortion, robberies and drug trafficking to "promote and further the activities of the Mongols gang," according to a 177-page indictment.
Among other things, prosecutors claim Cavazos was involved in negotiating with Mexican Mafia leaders over the collection of payments for drug trafficking in areas controlled by that gang and getting Mongol members to commit crimes against people who challenged the gang's authority.
The indictment said the group, which is mostly Latino, also attacked black people, committed robberies and stole motorcycles. The gang funded itself in part by stealing credit card account information, prosecutors said.
Authorities conducted a sweep last October in six states, rounding up members after an undercover investigation put federal agents inside the motorcycle club where they were accepted as full members.
A number of the 79 members indicted have already pleaded guilty to a variety of charges, but most of the plea agreements remained under seal. The ssociated Press has challenged a federal judge's decision to seal the documents.
Cavazos, a former medical technician at Los Angeles County-USC Medical Center, wrote a memoir of his life titled "Honor Few, Fear None: The Life and Times of a Mongol" that was published by HarperCollins last year.
On a Web site promoting the book, Cavazos wrote: "I would like to thank everyone who has stood by family and I. With your support and prayers, I am confident that I will see all of you on the outside again."
Absent from the site is the group's trademarked logo. U.S. District Judge Florence-Marie Cooper has barred members from wearing or distributing the image.
The Web site says, "By order of the federal attorney, image has been removed."
Prosecutors argue the logo should be forfeited because the trademarks were acquired and maintained by Cavazos while the gang was involved in criminal activity.
Assistant U.S. Attorney Steven Welk said in court documents that the Mongols have challenged the injunction.