U.S. District Judge Harry Leinenweber handed down the sentence Thursday in Chicago, calling Headley "a terrorist." He said he wanted to make sure he is "never in a position to commit a terrorist attack" again.
"There is little question that life imprisonment would be an appropriate punishment for Headley's incredibly serious crimes but for the significant value provided by his immediate and extensive cooperation," prosecutors said, seeking a lesser sentence of 30 to 35 years.
Indian External Affairs Minister Salman Khurshid said Headley deserved more than 35 years in prison.
India had sought to have Headley extradited so he could face trial in India, Voice of America reported.
Headley, 52, a U.S. citizen of Pakistani descent, admitted he attended training camps in Pakistan operated by the terrorist group Lashkar-e-Toiba on five occasions from 2002 to 2005, and traveled to India five times to conduct surveillance before the Mumbai massacre.
He faced possible life in the prison after pleading guilty to 12 counts for his role in planning the November 2008 Mumbai attack, which killed at least 160 people, including six Americans, and an aborted attack on a Danish newspaper related to the printing of a cartoon of the Prophet Muhammad.