(coexisting on another dimensional plane)
Following the loss of Terok Nor to the Terran Rebellion in 2372, Worf resolved to recapture the space station personally. He captured Garak, the former first officer of Terok Nor and the only Alliance official to escape the station alive. Worf initially accused Garak of being responsible for the loss of the station, but Garak deftly shifted the blame onto his former commander, Intendant Kira Nerys. Regardless of the blame, however, Worf had Garak locked in a dog collar and chain until the station was recaptured.
When the Battle of Terok Nor ended with an Alliance defeat, Worf raged that he had been betrayed – he believed there could be no other explanation for his loss. Garak took the opportunity to blame the Intendant for the rebels' victory. Worf agreed, and began plotting with Garak to capture the Intendant and obliterate the rebels. (DS9: "Shattered Mirror")
In 2375, Worf finally captured the Intendant, along with Zek, who had crossed over to the mirror universe to open a trade agreement between the two realities. At the Intendant's suggestion, the Regent offered to let Zek go in exchange for a cloaking device, a technology that the Alliance did not possess. However, Rom sabotaged the cloaking device so that it would disable all of the Regent's flagship's primary systems. With his ship completely disabled, the Regent was captured by the rebel warship ISS Defiant. (DS9: "The Emperor's New Cloak")
The mirror Worf was originally written in for a cameo appearance as a Klingon guard in DS9: "Crossover",  but his appearance was canceled when Michael Dorn (who played the counterpart) couldn't make the schedule (since he was still filming the last episodes of TNG at the time).
Mirror Worf is the only mirror universe counterpart of a TNG regular cast member to ever appear on screen.
Several mirror universe short stories, in books such as Obsidian Alliances and Shards and Shadows show that Worf, as regent, was in essence, the Klingon Chancellor and was in control of the Klingon Empire until he was captured by the Terran rebels, throwing the House of Worf (the House of Mogh in the primary universe) into disgrace. In his absence, he was deposed and replaced as Regent by Martok.
In Rise Like Lions, Worf is revealed to have been incarcerated in the Badlands following his capture. Miles O'Brien tried to interrogate him to find suitable strategic locations, but Worf told him to simply attack Qo'noS, an action both knew was impossible. After the rebellion won, O'Brien permitted Worf to leave. However, Worf chose to stab himself with a d'k tahg. As O'Brien had given him the dagger, Worf's final words were, "Thank you."
Prior to the establishment of the history of the televised mirror universe, a mirror version of Worf appeared in the novel Dark Mirror, depicting a mirror universe in which the Terran Empire had survived well into the 24th century with Klingons being one of their many slave races but had run out of worlds to conquer and now sought to expand into the primary universe. When Captain Jean-Luc Picard infiltrated the ISS Enterprise (ICC-1701-D) and posed as his counterpart, a brief moment of compassion that he showed to the mirror Worf, beaten and treated as a slave by his crewmates, prompted this Worf to realize his identity and to assist him later on. Picard encouraged the mirror Worf to prepare his people for a chance to strike back against the Empire when it inevitably collapsed.