She was born in 2350 to the Terran slaves Elias Vaughn and Ruriko Tenmei. Her father had been an officer in the Imperial Starfleet prior to the conquest of the Terran Republic, the successor state of the Terran Empire, by the Klingon-Cardassian Alliance in 2295. Her father eventually ended up on Bajor as the property of Jaro Essa's family. As Jaro was a member of the Bajoran dissident movement which sought the withdrawal of Bajor from the Alliance, he was treated humanely and was eventually given his freedom. However, the pretense that he was Jaro's slave was maintained in order to prevent the Alliance growing suspicious.
By 2377, her father was a blind and extremely frail centenarian who was confined to a hospital bed in a dissident enclave in Vekobet in the Kendra Province of Bajor which was passed off as a labour camp in which the vital metal uridium was processed by slaves for the Alliance. In reality, the slaves, who were composed of Terrans as well as Bolians and Tellarites, were agents of the dissident movement, as were their supposed Bajoran masters. Jaro, the camp's physician and one of the enclave's leaders, cared for him with Prynn's assistance. Prynn developed a close relationship with Opaka Sulan, another of the camp's leaders, to the point where Winn Adami occasionally referred to her as Opaka's daughter.
In January of that year, Prynn met her father's considerably healthier primary universe counterpart Commander Elias Vaughn, the first officer of Deep Space 9, who had travelled to the mirror universe with Captain Kira Nerys to warn the Terran Rebellion and their allies that Iliana Ghemor intended to discover the Bajoran wormhole and become the Emissary of the Prophets.
Shortly afterwards, Klingon forces from the IKS Negh'Var launched an attack on Vekobet. Recognising that he would slow down Prynn and the other dissidents, her father asked his counterpart to kill him. Commander Vaughn did so by shooting him with his phaser, which led to Prynn physically attacking him. However, she and Opaka were eventually able to escape through the Vekobet tunnels to another enclave.
Commander Vaughn was soon taken prisoner and transported to Terok Nor, which had been recaptured by the Alliance only hours earlier. During their captivity, Jaro and Winn told Commander Vaughn that they understood why he had killed his counterpart and that they forgave him for doing so. However, Commander Vaughn doubted that Prynn would be as forgiving. (DS9 novel: The Soul Key)
By 2378, she had joined the Terran Rebellion and was serving as the flight controller aboard the Defiant, having replaced Sam Bowers who was killed during the attack on the SoHcha shipyards. (ST - Mirror Universe novel: Rise Like Lions)