In the mirror universe, Rom was a Ferengi mercenary-for-hire during the latter half of the 24th century. He was the son of Ishka, the younger brother of Quark and the father of Nog. He lived in the Klingon-Cardassian Alliance underworld while Quark owned and operated a bar on Terok Nor. He often assisted his brother any way he could to free the station's captive Terran population.
All this changed when Quark was killed by Gul Elim Garak after his activities to free Terran slaves were exposed. Rom and Nog quickly joined Captain Benjamin Sisko's Terran Rebellion against the Alliance in the hope of avenging his death. He served in Sisko's cell of the rebel organization and quickly became a battle-hardened soldier. Although he inherited his brother's bar after his death, he never actually ran it due to his rebel activities.
When Sisko was killed during a mission to recruit Professor Jennifer Sisko, his estranged wife, Rom joined Julian Bashir's faction of Sisko's surviving troops and argued for an immediate military strike on Terok Nor to avenge their fallen leader. In contrast, Tuvok stressed caution as a full-scale attack on Terok Nor would have cost thousands of lives. Sisko (later revealed to be his primary universe counterpart) returned to end the argument and re-organized the plan to get to Professor Sisko.
Rom was sent to Terok Nor, posing as a double-agent for the Alliance, and informed Intendant Kira Nerys of Bajor and Garak of Sisko's survival. This was done to help Sisko get onto the station by being captured on purpose. Though Intendant Kira was totally convinced, Garak remained suspicious and secretly tracked Rom through the station and, when he discovered Rom communicating with Sisko with a subdermal transceiver, pushed him against the airlock where Rom's ship was docked and stabbed Rom through the gut, killing him and impaling him against the airlock door. Garak left his body suspended there for future warnings against rebellion. (DS9 episode: "Through the Looking Glass")
By late 2371, his mother Ishka was a successful businesswoman who owned a tunnel complex in an underground city on Archanis. She was an associate of the Alliance in spite of the fact that it was responsible for the executions of her two sons. (VOY - Mirror Universe short story: "Bitter Fruit")
Upon visiting the mirror universe in 2375, the Rom of the primary universe expressed sadness at the death of his counterpart. He told his brother Quark that he would have liked to have met himself. (DS9 episode: "The Emperor's New Cloak")