The Ivorian forward slotted home despite a bad first touch in the 73rd minute after Hanover, under interim coach Daniel Stendel following the sacking of Thomas Schaaf last week, had come from a goal down to lead 2-1.
Third-place Hertha, who have taken one point from their last two games, are on 49 points with five matches remaining.
They are four points ahead of Borussia Moenchengladbach and Bayer Leverkusen, who both have a game in hand and are in action on Saturday and Sunday respectively.
The top three teams qualify for the Champions League group stage with the fourth going into the competition's qualifying rounds.
Hanover are in last place on 18 points, nine short of Augsburg in the relegation playoff spot.
The hosts got off to a perfect start with Vedad Ibisevic scoring from close range after three minutes for his ninth goal of the season.
But Hanover, with former Under-19 coach Stendel making his Bundesliga debut, levelled with their first chance of the game 16 minutes later with Artur Sobiech converting after a superb assist from Hiroshi Kiyotake.
The visitors looked to have completed a remarkable turnaround when former Hertha youth player Manuel Schmiedebach's well-struck low drive went in off the post to stun the 40,000 crowd in the Olympic stadium.
Kalou came to the rescue, however, when he turned well to drill in for his 14th league goal of the season, although it looked as though he had wasted the chance following a bad first touch from Julian Schieber's pass.
Leaders Bayern Munich, five points clear at the top, take on VfB Stuttgart on Saturday. Second-placed Borussia Dortmund, on 67 points, travel to neighbouring Schalke 04 for their Ruhr valley derby on Sunday.
(Reporting by Karolos Grohmann; Editing by Toby Davis)
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