A computer hacker who helped the US government disrupt hundreds of cyber-attacks after his arrest has been set free.

Hector Xavier Monsegur was sentenced to time served by a federal judge on Tuesday, paving the way for his release after seven months in prison.

Prosecutors requested leniency, citing Monsegur's cooperation in helping the FBI foil cyber-attacks on the US Congress, NASA and other sensitive targets.

The FBI estimates Monsegur helped flag up at least 300 separate attacks and prevented millions of dollars in losses after switching sides.

He is credited with helping cripple 'Anonymous', the notorious group of hacktivists which has been behind a number of cyber-attacks on government agencies.

Monsegur "provided, in real time, information about then-ongoing computer hacks and vulnerabilities in significant computer systems" prosecutors said.

Monsegur first began hacking in a New York City apartment in the early 2000s, according to court papers. His aim then was to steal credit card information, then sell it or use it to pay his own bills.

He claimed in a 2011 interview with an online magazine that he decided to join forces with 'Anonymous' after the arrest of 'WikiLeaks' founder Julian Assange.

Using the alias 'Sabu', Monsegur led an 'Anonymous' splinter group called 'Lulz Security', or LulzSec. The group hacked the computer systems of Fox Television, Nintendo, PayPal and other businesses.

Prosecutors say the group was loosely affiliated with Jeremy Hammond, the FBI's most wanted cyber-criminal. Monsegur immediately agreed to cooperate when FBI agents went to his home in the summer of 2011.

He became a pariah after reports of him cooperating surfaced, prosecutors said. Hackers posted his personal information and he was approached on the street and threatened, they said.

The FBI had to relocate Monsegur and some of his family as a result.