Prosecutors said Headley's cooperation was a factor in his reduced sentence. Several victims' families objected to the leniency in Headley's sentencing.
Headley conducted scouting missions in Mumbai by videotaping and mapping various tourist locations in the city. Pakistani militant group Lashkar-e-Taiba used his information to coordinate attacks on targets that killed more than 160 people over three days in 2008.
The lone surviving attacker who was captured at the end of the attacks was executed in India last year.
The attackers arrived by boat on Nov. 26, 2008, carrying grenades and automatic weapons, and fanned out to hit multiple targets, crowded train station, a Jewish center and the hotel.
Former U.S. Attorney Patrick Fitzgerald spoke in court calling Headley's cooperation within 30 minutes of his 2009 arrest "unusual."
U.S. District Judge Harry Leinenweber said he considered the cooperation in imposing his sentence even though "the damage that was done was unfathomable."
Prosecutors say Headley, who was born in the U.S. to a Pakistani father and American mother, was motivated in part by his hatred of India going back to his childhood. He changed his birth name from Daood Gilani in 2006 so he could travel to and from India more easily to do reconnaissance without raising suspicions.
Prosecutors have recounted only in broad terms how Headley has shed light on the leadership, structure and possible targets of Lashkar-e-Taiba, which was believed to have ties to the Pakistani intelligence agency known as ISI. Headley has said his ISI contact was a "Major Iqbal," who was named in the indictment that charged Headley.
Prosecutors also have praised Headley for testifying against Tahawwur Rana, the Chicago businessman convicted of providing aid to Lashkar and backing a failed plot to attack a Danish newspaper for publishing depictions of the Prophet Muhammad. Rana, sentenced last week to 14 years in prison, claimed his friend Headley duped him.
The 12 counts Headley pleaded guilty to included conspiracy to commit murder in India and aiding and abetting in the murder of six Americans.
The Associated Press contributed to this report.
