The news comes as lithium prices remain at record highs as demand surges for the electric vehicle battery metal.
China's Ganfeng Lithium began the bidding war with a $353 million offer for Millennial in July.
It was then outbid in September by Contemporary Amperex Technology (CATL), the world's largest EV battery manufacturer, which offered $377 million, or $3.85 cash per share.
Lithium Americas (TSX: LAC) entered the fray this month with a 22.1% premium offer of $4.70 per share, payable in scrip and 0.1c per share in cash.
Millennial said yesterday CATL had not exercised its right to match LAC's offer and it had "terminated" that deal and entered a definitive arrangement agreement with LAC.
"This transaction is a rare opportunity to add a complementary lithium brine project and leverage our expertise developing Caucharí-Olaroz as the largest new lithium carbonate operation to come online in over 20 years," LAC president and CEO Jonathan Evans said.
Millennial's Cauchari East project is next to LAC's Cauchari, which is under development in partnership with Ganfeng.
The deal would also bolster LAC's growth pipeline while preserving cash on hand (US$482 million at September 30) for the development of its Thacker Pass lithium project in the US, the company said.
LAC said it had reimbursed Millennial the $20 million termination fee payable to Canada Brunp Contemporary for ending the CATL deal.
The acquisition is subject to regulatory and Millennial shareholder approval, with a vote expected in early January.
Millennial shareholders are set to hold 9.1% of LAC.
Millennial shares (TSXV: ML) closed down 1.9% yesterday to $4.63, valuing it at $453 million (US$356 million).
LAC shares rose 4% to $41.57, capitalising it at $4.99 billion (US$3.9 billion).