Jeremy Hammond on Tuesday pleaded guilty to hacking intelligence firm Stratfor to expose millions of emails, with some of the more revealing ones later published by WikiLeaks.
The Illinois native faces up to 10 years in prison when he is sentenced 6 September.
As part of his plea deal, Hammond, who was involved with Anonymous and offshoot LulzSec, accepted responsibility for infiltrating a number of other intelligence-related sites including equipment suppliers and police agencies.
He had been publicly accused of illegally accessing the Arizona Department of Public Safety.
In a Tuesday statement posted on his support website, FreeJeremy.net, Hammond said that by pleading guilty he is protected from being prosecuted for the other offenses.
Hammond has been held without bail in a New York jail since March 2012, often placed in solitary confinement and denied visitors.
"[E]ven if I was found not guilty at trial [in New York], the government claimed that there were eight other outstanding indictments against me from jurisdictions scattered throughout the country" he wrote. "If I had won this trial, I would likely have been shipped across the country to face new but similar charges in a different district. The process might have repeated indefinitely."
Hammond also stated that he was proud he shed light on the secret dealings of Stratfor, which serves major corporate and government agency clients.
A Rolling Stone article published in December said the emails referenced "nefarious and clandestine activities – from the US Government's monitoring of the Occupy movement to Stratfor's own role in compiling data on a variety of activist movements, including PETA, Wikileaks and even Anonymous itself."
"Now that I have pleaded guilty it is a relief to be able to say that I did work with Anonymous to hack Stratfor, among other websites," Hammond wrote.
"I did this because I believe people have a right to know what governments and corporations are doing behind closed doors. I did what I believe is right."
Prosecutors have said that in addition to the emails, Stratfor hackers also stole 60,000 credit card numbers from clients to make unauthorised charges totaling $700,000, in addition to the personal information, such as names and email addresses, of 860,000 people. Much of the charges reportedly were made on behalf of charities.
A Stratfor spokesperson did not immediately respond to a request for comment. The company has said the hack cost it millions, including nearly $2 million to settle a class-action lawsuit brought by members.
Supporters of Hammond have launched a petition asking that Judge Loretta Preska sentence Hammond to time served.
Authorities nabbed Hammond after he was given up by Anonymous-hacker-turned-informant Hector "Sabu" Monsegur. His LulzSec co-defendants recently were sentenced in the UK.