Davis took reliever Sean Doolittle deep in the eighth, when the Mets scored six runs with two outs on the way to a 6-1 win in New York.
"I think I saw him in the fifth or the sixth for the first time," Mets manager Mickey Callaway said. "He got here about the third. He Ubered over, got ready, came out, hit a homer, came inside, didn't know where the clubhouse was, how to get back in, had to ask some people, 'Which way do I go to get back in the clubhouse?' And just slapped high-fives going into the line. Yeah, whirlwind day for Raj; great at-bat."
Davis, 38, who grew up in Norwich, Connecticut, rooting for the Mets, said he found out he had been called up at about 5 p.m. He said he thought his manager was joking at first.
"It was a normal day when it started and then we took BP in Lehigh Valley, PA, in the cage and I found out I was coming up," Davis said. "Thought that the manager was playing around, playing a little joke. But I'm here."
The ride from Lehigh Valley took about two hours, and he said he got to know his driver, Jason, pretty well.
"We were both excited," Davis said.
The Mets are Davis' eighth team in a 13-year career. He's a .262 lifetime hitter, and led the league in steals in 2016 with 43. Last season, with the Cleveland Indians, he homered only once. He matched that Wednesday night.
The roster spot opened when the Mets put outfielder Brandon Nimmo on the 10-day injured list with a stiff neck.
Nimmo, who has struggled at the plate all season, had an MRI on Wednesday after experiencing a sharp pain in his neck earlier in the week. He is hitting .200 with three home runs and 14 RBIs in 130 at bats this season, far below last season's numbers.
Rajai Davis belts 3-run HR to give Mets insurance
Rajai Davis crushes one to left field and plates three runs, putting the Mets up 6-1 in the eighth.