In the Mirror Universe, Gul Elim Garak was the second-in-command of Terok Nor in the Klingon-Cardassian Alliance. He served under the command of Intendant Kira Nerys. He chafed under Kira’s command, constantly wishing to set brutal and violent examples at the slightest infraction made by the Terran slaves on the station.
Most of his time was spent hatching plans for the killing of the Intendant and taking over Terok Nor, unfortunately for him most of the plans were rather transparent and ended up failing. In 2370, the arrival of the Kira Nerys from the regular universe gave him a great opportunity. His plan was to kill the Intendant and have the regular Kira replace her, and then Kira would resign from the post and leave Garak in command. This particular plot failed because Kira left before he could have the Intendant “disappear”.
The next year, Garak wished to make an example of the leader of the Terran Rebellion, Benjamin Sisko. His hopes came crashing down when the Intendant was so happy to have her plaything back, that she told Garak to leave him alone. When Rom came forward with information on Sisko, Garak had him tortured into revealing the plan to have Jennifer Sisko removed from the station, he then had Rom executed.
In 2372, when Terok Nor was captured by rebels he was the only officer to escape from it. He was quickly captured and put in a dog collar for his disgrace, by Regent Worf. However he did manage to deflect blame for the loss at the Battle of Terok Nor onto the Intendant.
In 2375, the Regent finally caught uo with the Intendant, but managed to bargain her way put of punishment by suggesting the Regent use Zek to trade for a cloaking device (the Alliance didn’t have cloaking technology). Garak was disappointed that he wouldn’t get to execute the Intendant, but he did plan to execute Zek, Rom, and Quark instead. However, the Ferengi managed to disrupt Garak’s attention by saying that our Garak would have relished the chance to interrogate his prisoners and learn their secrets. The mirror Garak was more interested in just killing his prisoners, but the delay had stopped him just long enough for Ezri Tigan to sneak up and take the hypospray Garak was about to use on Quark. The hypospray had contained a ulcartic virus which would cause immense pain. Garak was left for dead.