Headley pleaded guilty to checking out targets for the Mumbai attack that left 166 Indians and foreigners dead, a US judge has ruled.
He entered a guilty plea on 12 counts in a Chicago court in 2010, reported BBC.
The sentencing of co-accused, Tahawwur Rana, who was convicted of aiding Pakistan-based Lashkar-e-Taiba, has been scheduled for Jan 15.
Headley had initially denied the charges but changed his plea to avoid the death penalty or extradition to India.
Nine of the 10 terrorists from Pakistan who sneaked into Mumbai were also killed while the tenth, Ajmal Amir Kasab, was arrested. He was hanged this month.
In pleading guilty, Headley admitted that he attended training camps in Pakistan operated by Lashkar-e-Taiba (LeT), a designated foreign terrorist organisation, on five separate occasions between 2002 and 2005.
Prosecutors said in late 2005, Headley received instructions from three members of LeT to travel to India to conduct surveillance.
Son of a Pakistani father and an American mother, he changed his name from Daood Gilani in 2006 to visit India five times to scout targets for the Nov 26 Mumbai attacks.
Headley, prosecutors said, has cooperated with the government since he was arrested Oct 3, 2009, and the plea agreement states that he "has provided substantial assistance to the criminal investigation, and also has provided information of significant intelligence value".