In the mirror universe, Seska was an influential Cardassian glinn under the command of Gul Evek on the warship Bak'rikan, which served the Klingon-Cardassian Alliance. During her term of service, she served as Evek's second-in-command, most trusted officer and secret lover. During their illicit relationship, she continuously pumped Evek for information when making love, gaining more insight into the Alliance's workings. Gradually, Seska and other Cardassians started to see the Alliance between their people and the Klingons as a spreading corruption that threatened Cardassia as the Klingons had more power over the Cardassians in government and military strength.
Seska decided to defect to the Terran Rebellion against the Alliance, as she believed if she helped the rebels drive the Klingons away, Cardassia's glory would be restored. She left the ship and reached rebel territory, but due to her Cardassian features, many rebels wanted to kill her. However, by bringing many Alliance secrets with her and pledging allegiance, she convinced the rebel leaders of her good intentions and served under Captain Chakotay on the Geronimo. Meanwhile, Evek's reputation and career was shattered by Seska's defection, making him a failure in the eyes of his superiors and fellow officers.
When Kes was captured by the Alliance, Seska was "convinced" by Tuvok of the importance of a telepath in the hands of the Alliance or Rebellion could decide the fate of the entire war and journeyed to the facilitiy in which Kes was being held in (which she learned the location of by using her contacts), but she, along with the rest of her group, were captured when Geronimo's engineer, Kate Janeway, turned out to be an Alliance spy.
Seska escaped, however, along with the rest of her group thanks to Harry Kim's assistance. Kim proceeded to massacre every Klingon soldier in the facility and Seska helped him sabotage the facility's anti-grav generators, causing the facility to begin crashing to the planet's surface. Seska and the group escaped in the Geronimo (excluding Chakotay who was separated from the group and killed while killing Janeway, and Kes who "ascended") back to the Badlands. Seska, while comforting Neelix, allowed him to join the Terran Rebellion's ranks and continued to fight against the Alliance. (VOY - Mirror Universe novel: The Mirror-Scaled Serpent)
By 2377, she was still serving aboard the Geronimo, which was under Neelix's command, as first officer. In 2378, she and Neelix were both killed during the Memory Omega led attack on the SoHcha shipyards. (ST - Mirror Universe novel: Rise Like Lions)