Gladbach, who narrowly avoided relegation last season, reached the halfway mark in fourth place with 33 points from 17 games, four behind leaders Bayern Munich and one adrift of Borussia Dortmund and Schalke 04.
Unbeaten at home in the league this season, Gladbach quickly took control when their top scorer Marco Reus skipped past a defender and found Herrmann, who slotted the ball past Christian Wetklo in the fifth minute
Wetklo made a double save to foil Juan Arango early in the second half, although it was not a totally convincing performance by the hosts who lived dangerously for much of the game.
Mainz coach Thomas Tuchel, whose side had a penalty turned down at the end of the first half, was sent off early in the second half after an angry protest to the referee.
Mainz are 14th with 18 points.
Relegation-threatened Kaiserslautern were held to a 1-1 home draw by Hannover 96 in Sunday's other match, where both teams failed to improve on their dismal home and away records respectively.
Mohammed Abdellaoue pounced on a mistake by Mathias Abel, who failed to cut out a long ball forward, and sent a low finish past Kevin Trapp in the 13th minute.
Adam Nemec levelled for the hosts midway through the second half after Olcay Sahan burst down the left and picked him out.
Kaiserslautern, with only two home wins in eight attempts, finished with 10 mean after Itay Schechter was sent off for lashing out at an opponent.
Hannover, who have taken only six away points, are seventh while Kaiserslautern are in 16th, the relegation play-off spot.
