Mathis caught for the Angels from 2005-11, while Lackey was with the club from 2002-09.
"He's a strike-thrower, looking to get ahead of you and put you away," Mathis said. "I was looking for something early."
Mathis' homer was a big moment for the Marlins, because they regained a 3-1 lead after the Cubs tied it at 1 in the sixth inning.
In each of Mathis' first two at-bats, he was retired on the first pitch. In the seventh inning, he belted an 0-1 fastball just far enough to give Miami a pair of runs.
According to Statcast™, Mathis' homer projected at 380 feet with an exit velocity of 103 mph.
"Mathis is my boy, too," Lackey said. "It's one of those deals, great swing by him, and it got out of the yard. For the most part, I felt I threw the ball pretty well. Good for him.
"He caught me quite a bit in Anaheim. It was a flip-a-coin kind of thing -- it was a platoon."
In his fourth season with the Marlins, Mathis is J.T. Realmuto's backup. He's known more for his defensive skills, but he came through against a pitcher with whom he is familiar with his second homer of the season.
"I was early a couple of at-bats before," Mathis said. "I got a pitch that third at-bat and did something with it."
There wasn't much for the Marlins to cheer about in Chicago, especially after losing three in a row for the first time since June 28-30.
A voice of reason on the club, Mathis' advice is: "Just leave it here in Chicago. Go into Denver, have the day off tomorrow to clear our heads and get back to work."
The Marlins open a weekend series against the Rockies at Coors Field on Friday.
Joe Frisaro has covered the Marlins for MLB.com since 2002. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.