Monsegur was said to be the "leader" of LulzSec, a group, formed in 2011, known for several high-profile hack attacks.
LulzSec was an offshoot of the Anonymous hacktivist movement, and took credit for hitting the websites of Sony Pictures Entertainment, Fox Broadcasting Company and the Public Broadcasting Service (PBS), among others.
When Monsegur was arrested later in the same year, he agreed to carry on operating in the hacking community, under FBI supervision.
Authorities said they were able to use his connections to prosecute and convict a number of hackers, including the "number one cybercriminal target in the world" at the time - Jeremy Hammond.
The FBI said that thanks to Monsegur, future high-profile attacks had been prevented.
Victims of the attacks were to include the US Armed Forces, the US Congress and Nasa as well as a "television network, a video game manufacturer and an electronics conglomerate".
The hacktivist community has reacted angrily to the sentencing.
Court documents made public this week described how Monsegur had been "approached on the street and threatened or menaced" once details of his cooperation had become known.
Reacting to the news today, Monsegur was described by some as a "traitor" and a "super snitch".