In 2267 the mirror universe versions of Uhura, Kirk, Montgomery Scott, and Leonard McCoy were all transposed with their prime universe counterparts when they beamed up from Halka during an ion storm. While their prime universe counterparts were more or less able to blend in, Uhura and her mirror universe compatriots were quickly exposed by the prime universe Spock, who had all four of them taken in to custody and placed in the brig.
Placed in the brig she shouted at the prime Spock, believing him to be a traitor to the Empire before Kirk waved for her and the others to be silent.
Spock placed Uhura and the mirror universe crew in the transporter chamber, knowing that sooner or later the prime universe counterparts would activate the transporter and beam everyone back to their home universes. When the transporter activated Uhura and her compatriots were beamed back to their home universe.
Uhura's fate following the transposition varied in various licensed literature.
Following her return home, Uhura continued to server on the Enterprise after Spock assassinated Kirk and assumed command. Uhura became more and more suspicious of Spock's actions in the months following her return. Unaware that Spock was monitoring her, Uhura met with Scott and McCoy and began plotting with them to kill Spock. Seconds after declaring that she would kill Spock herself, Uhura disappeared in a flash of light, vaporized by the Tantalus Field. This put a permanent end to her plans. Uhura was the first in a number of Kirk's other supporters who "disappeared" as Spock consolidated his power.
The Mirror Universe Saga
In the DC comics The Mirror Universe Saga Uhura remained on the Enterprise under Kirk's command as he made an effort to infiltrate the prime universe. She and the ISS Enterprise crew were captured by the prime universe Kirk and placed in stasis. The mirror universe crew were able to escape. She and everyone else on the ISS Enterprise died when it was attacked and destroyed by another Imperial vessel on orders from the Empire that believed them all guilty of treason.