Ryan Ackroyd's plea meant the court did not hear any evidence on the motivation behind the attacks.
Ackroyd used the fake persona of a 16-year-old girl named Kayla.
According to Gmanetwork, Ackroyd, 26, spoke only to identify himself and to enter his plea.
He was arrested in 2011 with three other British young men after a series of LulzSec attacks. LulzSec made international headlines in late 2010 when they launched what they called the "first cyber war" in retaliation for attempts to shut down the WikiLeaks website.
Mustafa Al-Bassam, 18, and Jake Davis, 20, had both pleaded guilty to two counts while Ryan Cleary, 21, had pleaded guilty to six counts including that he attacked Pentagon computers operated by the US Air Force.
Ackroyd faced four charges but pleaded guilty to just one. Prosecutors said they would not pursue the other charges.
Ackroyd and his three fellow hackers will be sentenced on May 14, Judge Deborah Taylor said.
Cleary, Al-Bassam and Davis admitted to launching a so-called distributed denial of service (DDoS) attacks - something that Ackroyd denied.
He admitted to hacking into computer systems, obtaining confidential data and redirecting legitimate website visitors to sites hosted by the hackers.
Ackroyd pleaded guilty to also hacking Britain's National Health Service, the US public broadcaster PBS and 20th Century Fox.
The three are bailed on the condition that they do not access the internet.
Cleary was indicted by a federal grand jury in Los Angeles last June, but US authorities have indicated they would not seek his extradition as he was being prosecuted in Britain on the same charges.