King was 1-1 with a 4.54 ERA in 55 appearances with the Nationals this season. Left-handed hitters were batting .161 against him this season.
It's a return to Milwaukee for King, who las played with the club from 2000 through 2002.
The Brewers also added some catching help by calling up Mike Rivera from Class AAA Nashville. Rivera was hitting .215 with 19 home runs and 61 RBI's in 96 games at Nashville.
Rivera and King are expected to join the Brewers tomorrow.