New York made the move after Lucas Duda went on the disabled list earlier this week with a stress fracture in his lower back and could miss two months.
Loney was released by the Tampa Bay Rays in early April. The Rays are responsible for most his salary. He was picked up by the Padres and has been playing for their Triple-A affiliate in El Paso.
The Mets only have to pay Loney the prorated portion of the major-league minimum for this year.
The trade was initially made known when Loney announced it on Twitter.
The 32-year-old batted .342 with two homers and 28 RBIs in 158 at-bats with El Paso.
Eric Campbell started four consecutive games at first base for the Mets since Duda was injured. However, Campbell is only hitting .182 with one home run and five RBIs in 55 at-bats this year.
Infielder Wilmer Flores is expected to return from the disabled list soon and could platoon at first base with Loney. Flores bats right-handed while Loney is a lefty.