Both sides are fighting relegation and Bochum were ready to stay out of the relegation zone at least until Saturday afternoon, but conceded the equaliser in the sixth minute of added time.
Former Mainz midfielder Kevin Stöger broke the deadlock for Bochum in the 21st after Keven Schlotterbeck was fouled inside the box.
Mainz, who were yet to have a shot on target, got the equaliser out of nowhere when Schlotterbeck was again under the spotlight and was accidentally hit by the ball, firing against his own net in the 59th.
But Bochum remained the more threatening team on the pitch and were rewarded in the 82nd as Schlotterbeck redeemed himself by finishing off a corner.
What seemed to be the winner, however, was not enough. Mainz got a late free-kick that was hoisted into the box. The ball ended up running loose for Krauß who smashed it on target with the help of a deflection.
"(It feels) unpleasant. It hurts a lot when you concede the equaliser shortly before the end because I think we deserved the win," Stöger told broadcaster DAZN.
"The next days will be disappointing and bitter, but we move on."
Schlotterbeck stressed there were still "reasons to smile because in the end we played a good game during 90 minutes. That shows what VfL Bochum are made of."
But he added it was also "hard to find the words" after conceding the equaliser so late.
Bochum are 16th, in the relegation play-offs spot, while Mainz sit bottom of the table.
Champions Bayern Munich will have Manuel Neuer back between the posts when they host Darmstadt on Saturday, while leaders Bayer Leverkusen welcome Freiburg on Sunday.
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