Acquired in a trade with the Braves on Wednesday, Johnson was officially activated prior to Friday night's game against the Brewers. The Mets cleared room on the roster by optioning infielder Ty Kelly to Triple-A Las Vegas.
Johnson was immediately thrown into the lineup at second base in place of Walker, who was sore a day after taking a 107-mph ground ball to the chest in the eighth inning of New York's 5-2 win on Thursday.
"[Walker's] sore, but he said he could play," manager Terry Collins said. "I was going to give him a day this weekend anyway, so we'll try to use today to see if he can get the soreness out of there."
With third baseman David Wright and first baseman Lucas Duda on the disabled list, the versatile Johnson provides the Mets coverage as a spot starter or off the bench.
Johnson primarily played second base for the Braves this season, but he has played six positions in his career.
"That's exactly how he's going to be used," Collins said. "He's going to spell guys. He gives us some flexibility. He's played a lot of left field. We have some double-switch options. He's a good bat off the bench. As we know, anything can happen, and you look up and you are getting a lot more playing time than expected."
Having gone from the Braves to the Mets last July as well, Johnson joins Chad Kreuter as the only players in Major League history to be traded between the same two teams in consecutive seasons. Kreuter was traded from the White Sox to the Angels in 1997 and 1998.
Johnson hit .250 with five home runs and 13 RBIs in 49 games for the Mets last season.
"It is obviously a good situation," Johnson said. "I know what I'm coming into. I'm way more comfortable. Them knowing me too is also a key. You don't have to feel any pressure about proving to be this or that. You just do the same thing, play hard and hope to do stuff to contribute to some wins."
Andrew Gruman is a contributor to MLB.com based in Milwaukee.