Monsegur's role combating cyber attacks from the Anonymous hacker collective and LulzSec group was revealed during his sentencing hearing on Tuesday.
During the hearing, prosecutors praised Monsegur for his "extraordinary" contributions combating the threats and recommended he receive a sentence of time served.
"Through Monsegur's co-operation, the FBI was able to thwart or mitigate at least 300 separate hacks. The amount of loss prevented by Monsegur's actions is difficult to fully quantify, but even a conservative estimate would yield a loss prevention figure in the millions of dollars," read the document.
"In light of the foregoing facts, the government respectfully requests that, pursuant to Section 5K1.1 of the guidelines, the court grant the defendant a substantial downward departure at sentencing."
Monsegur was exposed as an informant for the FBI in 2012, when law enforcement agents arrested key members of the LulzSec hacker group, including Ryan Ackroyd, aka "Kayla"; Jake Davis, aka "Topiary"; Darren Martyn, known as "pwnsauce"; Donncha O'Cearrbhail, called "palladium"; and Jeremy Hammond, aka "Anachaos".
During its heyday, LulzSec successfully mounted cyber attacks on numerous high-profile targets, including the UK Serious Organised Crime Agency (SOCA), Fox Television, Nintendo and Sony.
The documents revealed Monsegur's co-operation with the Feds led to threats on himself and his family from within the hacker community.
"Monsegur repeatedly was approached on the street and threatened or menaced about his co-operation once it publicly became known," read the document.
"During the course of his co-operation, the threat to Monsegur and his family became severe enough that the FBI relocated Monsegur and certain of his family members."
Monsegur originally pleaded guilty to multiple counts of computer misuse and fraud on 15 August 2011. Monsegur's final sentencing was delayed in February 2013 for an unknown reason.
He is yet to receive his final sentence, though he has already served seven months for his involvement in cyber attacks.