A Kiwi debutant in Denver who made his NRL debut with the Warriors in 2009, Ah Mau has impressed this year off the bench for the Dragons.
However he made clear to the Saints earlier this year he wanted to return home to New Zealand, and knocked back an offer from the Red V to sign with the Warriors until at least the end of 2021.
"Leeson will be a wonderful addition to our squad from next season," Warriors coach Stephen Kearney said.
"He's a big forward who is consistently one of the Dragons' best week in, week out.
