Their relationship continued from 1990 to 1995 when Showalter coached and then managed Mattingly during his final six seasons with the Yankees, culminating in New York's first playoff appearance since 1981.
These days, both are presiding over last-place teams.
The Marlins opened the series with a 2-0 victory and followed by getting two homers to center field from J.T. Realmuto in Saturday's 5-4 win.
Realmuto drove in four runs and posted his fourth multi-homer game for the Marlins (28-43), who are 8-4 in their last 12 games and within striking distance of the fourth-place New York Mets in the National League East.
"JT's been great on both sides defensively and offensively calling the game, everything about him's been great," Mattingly told reporters.
Realmuto missed the first three weeks of the season with a bruised back but is hitting .309 with nine home runs and 26 RBIs.
Meanwhile, Baltimore is on its longest losing streak since also dropping nine straight in July 2011 when Showalter was in his first full season at the helm.
The Orioles (19-50) also tied the 2007 club record with their 11th straight home loss. Their next setback will give them a double-digit losing streak for the first time since May 26-June 5, 2010 -- two months before Showalter took the job.
Manny Machado is doing his part for the worst team in baseball by hitting .308 with 18 homers and 52 RBIs. He drove in two runs Saturday and Jonathan Schoop homered.
Chris Davis, who is batting .150 with 86 strikeouts, has yet to see an at-bat in the series and could sit again Sunday for the Orioles, who own a .225 team average.
"I'd love to get back Chris Davis that we all know he's capable of," Showalter told reporters Friday. "It hasn't been there this year, but with kind of a new approach and some new things you're trying, all of a sudden you do it."
The Orioles lost 50 of their first 68 games for the first time since 1988 when they opened with 21 straight losses. Baltimore also is 28 1/2 games behind the New York Yankees in the American League East.
On a team with a 4.88 ERA, Dylan Bundy (4-7, 3.66 ERA) is the closest thing to an ace for Baltimore. Bundy turned in eight scoreless innings when he allowed three hits Monday in Baltimore's 2-0 loss in 12 innings to the Boston Red Sox.
Bundy's last start occurred after he pitched seven sharp innings in a 1-0 victory over the New York Mets on June 6 at Citi Field.
Monday was the fourth time this season that Bundy did not allow a run.. He has not given up a run in 17 innings since Washington's Mark Reynolds homered off him in the fourth inning on May 29.
"You could say that (it's a good run)," Bundy told reporters. "Body feels good, arm feels great. Just taking the ball every five days and trying to give our team a chance to win."
Sunday is Bundy's first career appearance against the Marlins. In seven (six starts) interleague games, he is 4-2 with a 2.39 ERA.
Trevor Richards (1-3, 4.41 ERA) will make his eighth career start for Miami after his first career victory.
Richards picked up his first win Tuesday when he allowed one run and two hits in six innings of a 3-1 win over the San Francisco Giants.
On a team looking for young pitching, Richards has been among the more positive developments. Richards, undrafted out of Drury College, was Miami's minor league pitcher of the year in 2017 in at two levels and made his major league debut April 2.
"He's been a good story, he's thrown the ball well," Mattingly told reporters. "Obviously, pitcher of the year last year in the minor leagues, really good reports on his makeup and how he works, competes, and pays attention, so it's good to see guys like that have success."