Byrd, 35, has 96 home runs in a Major League career that's in its 12th season, and seven of those have come with the bases jammed.
"You don't always come through, but it's just one of those things. When you do, it feels good," Byrd said. "I know they really weren't going to pitch to David [Wright]. Whoever's hitting behind him, at some point you got to come through to try to get him more pitches in the series or later on in the season. It was just nice to get a hit there."
Byrd hit his grand slam off reliever Jake Dunning during a five-run eighth inning when the Mets snapped a 5-5 tie and built a 10-5 lead.
"He's been a huge, huge force in the middle," Mets manager Terry Collins said. "When we moved Ike [Davis] out, we had to have somebody drive in some runs behind David because they were just pitching around him, pitching around him. We needed somebody to step in there and get some big hits, and Marlon has done that for us. He's really been a good find for us. I think playing every day has helped him get back in the swing of things. I think he's out to show everybody he can still play."
The Mets signed Byrd as a Minor League free agent on Feb. 1. Last year he was released by the Red Sox on June 14 then suspended June 25 for 50 games for violating MLB's drug policy. Byrd was reinstated Aug. 21 but didn't land another Major League job and wound up playing in the Mexican League during the offseason.
"Baseball's fun, baseball's fun," Byrd said. "I've always loved the game. You have a kind of renewed love when you go down to Mexico. Playing down there, you want to get back to the big leagues. I'm just enjoying myself. That's it. Going out there, having fun, playing hard and trying to help the team win every day."
Byrd helped the Mets win their third straight game, including two against the Giants. They'll go for the series sweep Wednesday afternoon.
Mets backup catcher Anthony Recker snapped a 3-3 tie in the sixth, launching Barry Zito's first pitch into the left-field seats for a two-run shot.
"It was my job to hold them down and I wasn't able to do that," Zito said.
Josh Satin led off the inning with a walk, then with one out, Recker crushed Zito's 80 mph cutter. Recker entered the game batting just .164, but five of his 12 hits in just 72 at-bats are home runs.
"Lately I've been swinging the bat pretty well," Recker said. "I can get kind of streaky with home runs at times. It seems like I'm in a good streak right now. Hopefully I can keep it going."
With one out in the eighth, Omar Quintanilla hit an infield single off reliever Jose Mijares and moved to second on Juan Lagaras' pinch-hit single off Jeremy Affeldt. After both runners advanced on a wild pitch, Affeldt walked Eric Young Jr., loading the bases.
Second baseman Daniel Murphy drove in Quintanilla with a sacrifice fly. After Affeldt walked Wright to reload the bases, Dunning came on to face Byrd, who sent his first pitch into the left-field seats for his 14th home run of the season. It was his first grand slam since Sept. 26, 2009, against Tampa Bay.
"Marlon's been unbelievable," Collins said. "And Dan Murphy's at-bat -- he's down in the count, Affeldt's awfully tough on lefties, and I thought Dan really had a good at-bat to get us a run. And Marlon just goes up and does what he does. He's dangerous and got a hanging slider and made a big difference."
Mets right-hander Dillon Gee came into the game on a roll, going 4-1 with a 2.47 ERA over his previous seven outings with six quality starts. Against the Giants, Gee struggled with his command much of the night, allowing five earned runs on nine hits while walking five over 6 2/3 innings. The Giants put at least one runner on base in every inning against Gee.
Before facing the Mets, Zito was 4-1 with a 1.98 ERA in 10 starts at AT&T Park, but he lasted just six innings Tuesday night, allowing five earned runs on seven hits.