The meeting between Mr Mattis and Prime Minister Modi came ahead of the latter's maiden meeting with US President Donald Trump at the White House. Mr Mattis met Mr Modi at the Hotel Willard Intercontinental, where the Prime Minister is staying.
Senior Indian officials, including National Security Advisor Ajit Doval were present at the meeting.
While the deal on the 22 Guardian worth between $ 2-3 billion has not been formally announced, it is considered to be a "game changer" for the US-India relations as it operationalises the status of "major defence partner".
According to General Atomics, the Predator Guardian UAV, a variant of the Predator B, can be used for wide-area, long-endurance maritime intelligence, surveillance and reconnaissance missions. The Indian Navy had made the request for this intelligence, surveillance and reconnaissance (ISR) platform last year.
The Guardian has cutting edge technologies that do not do not exist in the current Indian Navy arsenal.
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.