Christian Schulz drilled Hanover, who are fourth in the table with 13 points from eight games, ahead in the 23rd minute after the visitors failed to clear a corner.
Hanover's Ivorian striker Didier Ya Konan was then taken to hospital after twisting his ankle on the stroke of halftime.
Hertha went on the attack after the interval and created their best chance when Anis Ben Hatira hit the post after 58 minutes.
With the visiting fans chanting his name, mercurial midfielder Ronny went on as an 80th-minute substitute.
Within a minute he unleashed an unstoppable free kick that sailed over the wall and into the top corner of the net to put fifth-placed Hertha on 12 points.
Hanover, who were lacklustre for much of the game, could have snatched a last-minute winner but keeper Thomas Kraft saved a point-blank header from Artur Sobiech.
On Saturday, leaders Borussia Dortmund (19 points) travel to Borussia Moenchengladbach while second-placed Bayern Munich (also on 19) visit Bayer Leverkusen who are third.