A man who Tahawwur Rana, a Chicago businessman accused of helping plan the attacks.
Mr Rana is accused of providing Headley with a cover to scout attack locations.
Mr Rana has pleaded not guilty. His lawyers say he was tricked by Headley, a former friend.
More than 160 people were killed in November 2008 when a group of 10 men stormed a train station, hotels and cafes and a Jewish centre, shooting and throwing bombs.
At the opening on Monday of Mr Rana’s trial, Headley testified that Pakistan’s Inter-Services Intelligence agency (ISI) and militant group Lashkar-e-Taiba (LeT) coordinated with each other.
The ISI provided military and moral support to the group, he said.
He said a Pakistani agent paid him $25,000 (Â£15,500) for the operation. Headley has separately said he was trained by LeT.
Pakistan believes Headley is an unreliable witness and analysts expect Islamabad to flatly deny alleged links between militant groups and the Pakistani secret intelligence service.
‘Friends with a terrorist’
Mr Rana denies the 12 charges that have been levelled against him.
His lawyers say he was tricked by Headley, a longtime friend from their days at a Pakistani military school. Mr Rana was arrested in 2009, and if convicted, he faces a life sentence….