Mainz, with five wins and two draws from their seven previous matches, should have scored several goals in a dominant first half, with Jean-Paul Boetius coming close three times, including hitting the post, and Adam Szalai wasting another golden chance in front of goal.
It was Hertha, without a game since April 10, who struck first with Loucas Tousart's darting header in the 36th minute and their first chance in the game.
The hosts drew level four minutes later when Phillipp Mwene beautifully curled his shot from outside the box into the top corner.
Hertha are 17th with 27 points from 29 matches and have two games in hand. Cologne are 16th in the standings and occupy the relegation playoff spot with 29 points from 31 games. Bottom club Schalke 04 are already relegated.
Mainz, 12th in the table, have 35 points and are almost certain to stay up.