Johnson was obtained Wednesday for minor league right-hander Akeel Morris and cash. The teams had been discussing the deal off and on for the past week, Mets assistant general manager John Ricco said.
Johnson becomes the second player in major league history to be traded from one team to another in consecutive seasons, according to the Elias Sports Bureau. Chad Kreuter was traded from the White Sox to the Angels in 1997 and again in 1998.
"Obviously knowing Kelly last year, and knowing that he was out there on a team that wasn't playing very well, we kept our eye on him," Ricco said. "Even though he's not having a great year this year, he's a guy we kept our eye on because of the versatility, and we know him. He's a veteran presence."
Johnson intends to join the Mets on Friday in Milwaukee.
"It's an ideal move for me, because I was with the Mets last year," Johnson told the Braves' web site. "It's exciting, and hopefully we can go one step further this year."
With David Wright, Lucas Duda and Travis d'Arnaud all sidelined for prolonged periods with injuries, the Mets have struggled offensively. They rank last in the majors since May 12 in on-base percentage (.282) and runs per game (2.8).
Manager Terry Collins expects that the lefty-hitting Johnson will see some time at third base complementing righty-hitting Wilmer Flores. Johnson primarily will be a pinch hitter, but also should see time at first and second base. Collins noted that Johnson again can provide veteran clubhouse leadership, too.
Collins acknowledged that the Mets have discussed moving Neil Walker to third base and promoting second-base prospect Dilson Herrera. But the manager added that despite Walker being on board with that concept, it is not in the immediate plans.
The Mets had acquired Johnson and fellow infielder Juan Uribe from the Braves last July. That move, coupled with the acquisition of Yoenis Cespedes a week later, allowed the Mets to become the first team in major league history to rank last in runs scored on July 31 and reach the World Series.
Johnson, 34, re-signed with the Braves in January for one year at $2 million. He was hitting .215 with one homer and 10 RBIs in 121 at-bats this season.
The Mets had considered re-signing Johnson last offseason, but went in other directions.
"We ended up bringing in some other players and it wasn't the obvious fit, but now with the injuries we have some spots open," Ricco said.
Once Duda landed on the DL with a stress fracture in his lower back, the Mets traded for veteran first baseman James Loney on May 28.
On swiftly making two moves weeks before the trade deadline, Ricco said: "It's no secret, we have aspirations of winning the whole thing this year. So we're not going to sit around and wait. We know what this team is. We know what this team can be. And we're going to give it every chance to get there."
Ricco suggested that the Mets have the remaining prospects to swing a significant deal before the trade deadline if desirable, as was the case last season. The signature move in 2015 involved trading highly regarded pitching prospect Michael Fulmer to the Detroit Tigers on July 31 for Cespedes.
"Certainly we made a lot of trades last year, but we still have some pretty good players in the system," Ricco said. "I'm not saying we're going to trade them all. But I think we have assets to get some deals done."