David Headley gave details of the planning to a court in Mumbai on Monday through a video link from a prison in the US.
Headley, 52, pleaded guilty and co-operated with the US to avoid the death penalty and extradition to India.
More than 160 people were killed by gunmen in the November 2008 attack.
Headley is serving a 35-year jail term in the US for his role in the attacks.
Indian prosecutor Ujjwal Nikam said that "this was for the first time that a foreign terrorist" had appeared through a video link in an Indian court to testify.
"This is a very crucial case... I am absolutely satisfied as to what David Headley has revealed in today's deposition. I may quiz Headley on certain aspects, which were never asked by the FBI," he added.
Mr Nikam added that Headley's questioning would continue on Tuesday.
The Mumbai court gave him a conditional pardon in December and allowed him to turn witness.
Headley was sentenced in the US in 2013 on 12 counts, including conspiracy to aid militants from the Pakistani group Lashkar-e-Taiba (LeT) which India blames for carrying out the attacks.
After initially denying the charges, he eventually pleaded guilty and co-operated with the US to avoid the death penalty and extradition to India.
He admitted to scouting potential target locations in Mumbai ahead of the attacks.
Headley was born Daood Gilani to a Pakistani father and American mother but changed his name to David Coleman Headley in 2006 "to present himself in India as an American who was neither Muslim nor Pakistani", US prosecutors had said.
Headley is alleged to have told US prosecutors that he had been working with LeT since 2002.
He was arrested by FBI agents in Chicago in October 2009 while trying to board a plane for Philadelphia.
The 60-hour assault on Mumbai began on 26 November 2008. Attacks on the railway station, luxury hotels and a Jewish cultural centre claimed 166 lives. Nine gunmen were also killed.
The only attacker captured alive, Pakistani Mohammad Ajmal Amir Qasab, was executed in India in 2012.
A US man convicted for his role in the 2008 Mumbai attacks has told an Indian court that he visited Mumbai seven times in advance to gather information.