Mike Stanton hit a two-run double and slumping Mike Cameron followed with an RBI single for Florida, on the cusp of escaping the NL East cellar.
The Marlins improved to 15-9 under the 80-year-old McKeon, who took over after Edwin Rodriguez abruptly resigned late in the team's 1-19 June swoon.
"We never thought we were going to the park to lose a game, but Jack kind of adds a little life to the dugout, keeps it loose in there with his sayings, and does a good job of managing the game," outfielder Logan Morrison said.
In the makeup of a May 17 rainout, Hensley (1-2) allowed one hit over five scoreless innings against a depleted Mets lineup. It was his 41st big-league start - but first since July 24, 2008, for San Diego. The righthander made 20 relief appearances this season before a sprained shoulder sidelined him June 1.
Willie Harris' first-inning double was the only hit given up by Hensley, who struck out three and walked two in an 84-pitch outing. Michael Dunn, Edward Mujica, Randy Choate and Leo Nunez completed the three-hitter.
As long as he's effective, Florida plans to keep Hensley in the rotation, which is minus injured ace Josh Johnson.
"It's definitely a different beast than coming out of the bullpen and I haven't done it in a while, so I was pretty fired up that first inning," Hensley said. "But as the game started going on I kind of settled in and relaxed a little bit more."
The Mets expect to get star shortstop Jose Reyes (hamstring) and slugger Carlos Beltran (flu) back today, with David Wright (back) soon to follow this week.
But on this night, they looked listless and inept at the plate.
