Bryant Neal Vinas has already served about 8 and a half years in prison after pleading guilty in January 2009 to a variety of terrorism charges. He could have faced life in prison, according to federal sentencing guidelines.
Vinas's defense attorneys had argued that he should be released because he has provided "exceptional cooperation" with the US government since his arrest in Pakistan.
"However, we can proudly state that Mr. Vinas took the worst experience in his life and turned himself into one of America's greatest weapons against al Qaeda," attorneys Michael Bachrach and Steve Zissou wrote in a court filing Friday.
"Stated simply, he saved lives, and he helped the United States government substantially dismantle what had been the greatest threat to our nation and to our Western allies."
Prosecutors agreed that Vinas had provided "extremely substantial" intelligence against the terrorist group.
"He may have been the single most valuable witness against al Qaeda," prosecutors said.
At his court appearance, the 34-year-old Vinas accepted responsibility and apologized to the court.
"I would like to express my deepest apologies to the court," he said. "I would like to turn a bad thing into a good thing."
'American al Qaeda'
Vinas's remarkable tale was the subject of a 2010 CNN documentary "American al Qaeda
Born in Long Island to South American parents, Vinas became radicalized and traveled to the Afghanistan-Pakistan border in 2007 to join al Qaeda and fight American troops.
While there, Vinas pitched senior al Qaeda figures on the idea of bombing the LIRR and a WalMart store, he later told US officials. The bombings were never carried out.
As CNN has reported, his capture in Pakistan in November 2008 was kept secret for eight months because of the highly sensitive information he started giving US authorities within days after they caught him.
In 2010, a senior US counterterrorism official told CNN that Vinas had provided "extremely helpful information" in targeting al Qaeda operatives in the tribal areas of Pakistan.
"Just by talking about what he saw, telling us what houses and streets operatives were living in, identifying how al Qaeda runs its courier networks, he has provided us priceless information," the senior official told CNN. "This was actionable intelligence."