Cody Kretsinger, 24, member of Anonymous offshoot LulzSec has pleaded guilty to participating in an extensive hack of Sony Pictures Entertainment in 2011.
Kretsinger, pleaded guilty in a California federal court to one count each of conspiracy and unauthorised impairment of a protected computer in a deal with prosecutors. LulzSec, an offshoot of the international hacking group Anonymous, has taken credit for hacking government and private sector websites.
"I joined LulzSec, your honour, at which point we gained access to the Sony Pictures website," Kretsinger, who went by the hacking moniker "Recursion," told the judge after entering his guilty plea.
Kretsinger testified that he gave the information he got from the Sony site to other members of LulzSec, who then posted it onto the group's website and on Twitter.
Kretsinger and other LulzSec hackers, including those known as "Sabu" and "Topiary," stole the personal information of thousands of people after launching an "SQL injection" attack on the website, and ultimately caused Sony Pictures Entertainment over $600,000 (Â£380,000) in damages, the court heard.
Sentencing is scheduled for 26 July. Kretsinger faces up to 15 years in prison but it is thought likely he will receive a much lower sentence.
Anonymous and its offshoots including LulzSec and AntiSec focused initially on fighting attempts at internet regulation and the blocking of free illegal downloads, but have since taken on other targets including Scientology and the global banking system.
Anonymous â€“ and LulzSec in particular â€“ became notorious in late 2010 when they launched what they called the "first cyber war" in retaliation for attempts to shut down the WikiLeaks website.
They attacked websites including MasterCard.com which had tried to block payments to WikiLeaks after apparent pressure from the US government following the release of thousands of diplomatic cables.