Walker, 31, hit.264 with 10 home runs and 36 RBIs in 73 games with NY this season.
With Walker heading to Milwaukee, he has exclusively played in the National League over his career. NY acquired Walker from Pittsburgh in a trade for pitcher Jon Niese in December 2015.
Addison Reed, Lucas Duda and Jay Bruce were all traded by the Mets in recent weeks, saving the club about $9 million.
Overall though, Walker has actually had a decent second year in Queens, so the Brewers will be picking a very talented infielder.
The Mets enter the weekend in the logjam of the NL East and nine games under.500 at 52-61. An inconsistent offense and failure to produce with runners in scoring position have pushed the Brewers back to even at 59-59 entering Saturday, with Milwaukee looking to snap a six-game losing streak. The club's pitching staff has been decimated by injuries, with six starters now on the disabled list.