Headley, 52, Washington born son of a Pakistani diplomat and an American mother, "not only worked at the direction of Lashkar-e-Taiba for years, but also with members of Al Qaeda," prosecutors told US District Judge Harry Leinenweber.
Leinenweber Thursday sentenced Headley to 35 years in prison in line with the prosecution recommendations, which had suggested a 30-35 year term citing "significant cooperation" provided by Headley to US government's efforts to combat terrorism.
Ahead of Headley's sentencing, Gary Shapiro, the acting US Attorney in Chicago, told the court in a memo that "Headley played an essential role in the planning of a horrific terrorist attack" in Mumbai.
David Headley in Chicago court
Starting in 2002 and continuing to 2005, Headley attended five separate Lashkar training camps where he was indoctrinated on the merits of waging jihad and trained in combat skills, the memo noted.
At the instance of LeT leaders, he changed his given name of Dawood Gilani and enlisted the assistance of his friend, Tahawwur Hussain Rana to travel to and stay in Mumbai without detection starting in 2006 and continuing to 2008.
Headley provided video of and intelligence about the locations that later were targeted during the Mumbai attacks, Shapiro said.
Headley also assisted in plotting out a portion of the nautical route that the attackers would take, and recommended the landing point that they later used to enter Mumbai without being detected.
"Headley played a supporting role to the attacks, but an essential one that unquestionably contributed to the mass murder that took place," Shapiro said.
Before the Mumbai attacks took place, Headley was instructed by his Lashkar handler to travel to Denmark to perform surveillance for yet another attack - on the Danish newspaper Jyllands-Posten, he said.
After seeing what took place in Mumbai in November 2008, Headley travelled to Denmark in January 2009. There, he gained entry to two separate facilities for the newspaper, using the same cover story provided to him by Rana that he used in Mumbai.
In February and May 2009, he met with Ilyas Kashmiri, who encouraged Headley to break away from Lashkar and plan the attacks in Denmark with his group.
In sum, Headley worked to advance the violent goals of terrorist organizations and terrorist attacks for approximately seven years, the memo noted.