Early in her career, Lester served in Starfleet alongside James T. Kirk, with whom she presumably had a romantic relationship. She left Starfleet, perceiving a level of sexism in the organization which she believed would prevent her ever obtaining her goal of commanding a starship.
By 2269 Lester was leading an expedition to the planet Camus II, where she discovered a device capable of transferring a consciousness between bodies. Seeing this as an opportunity, she went about learning the intricacies of Starfleet operations before conspiring with the expedition's physician Arthur Coleman to murder the rest of the expedition, as a means of drawing the USS Enterprise to the planet when they then sent a distress call.
The ruse worked and a landing party from the Enterprise transported down to investigate. Lester used the alien device to transfer her and Kirk's consciousnesses. Lester successfully played Kirk for a time, but eventually the crew grew suspicious. When Lester made an attempt to kill Kirk in her old body, their consciousnesses transferred back. (TOS episode: "Turnabout Intruder")
As a lieutenant in 2264, she served on the UESS Enterprise, and was one of the security team to board the Romulan ship where T'Pol had been held hostage. (TOS - Myriad Universes novel: A Less Perfect Union)