The Marlins confirmed the deal Wednesday.
The 35-year-old Mientkiewicz is attempting a comeback from a separated shoulder that required surgery last season and limited him to 18 at-bats for the Los Angeles Dodgers. He was with them in spring training this year but failed to make the team.
Mientkiewicz broke into the major leagues in 1998 with Minnesota and has a lifetime average of .271 in 1,087 games. He won a Gold Glove in 2001 with the Twins.
Mientkiewicz went to high school in Miami with Yankees star Alex Rodriguez.