The mirror-universe counterpart of Worf rules the Klingon-Cardassian Alliance as its Regent, leading its fleet against the Terran Rebellion in person and losing twice to the rebels of Terok Nor.
In the mirror universe Worf never joined any Starfleet. He is the Regent of the Klingon-Cardassian Alliance, a loud, impatient ruler who commands from the bridge of an enormous flagship and treats his subordinates as property.
He first appears in Shattered Mirror, leading a fleet to retake Terok Nor from the Terran rebels and keeping the mirror Garak on a chain as punishment for losing the station. His attack is broken by the rebels' newly built Defiant, flown by the visiting Benjamin Sisko.
He returns in The Emperor's New Cloak, holding Grand Nagus Zek hostage in exchange for a cloaking device from the other universe. Quark and Rom deliver one, but Rom has sabotaged the installation, the flagship loses power in battle and the Regent is forced to surrender to Smiley O'Brien.