Ryan Ackroyd, 26, admitted being part of the group that emerged as a splinter-group of the Anonymous hacking collective in 2011.
Lulzsec targeted Britain's National Health Service and its Serious Organized Crime Agency, as well as military and security networks in the United States.
Ackroyd pleaded guilty Monday to one charge of carrying out an unauthorized act to impair the operation of a computer, the BBC reported.
As part of the plea agreement, he will not face trial on another count of operating a "denial of service attack."
He will be sentenced next month along with three other members who have pleaded guilty to various hacking charges, authorities said.