Early in his career, Valdore served as a senator in the Romulan Senate, where he once considered Senator Vrax a friend. Valdore made the mistake of challenging the precept of unlimited expansion. Upon asking the question, "Is conquest truly the best course for our people?", he was expelled from the Senate. From that point on, he vowed never to repeat his error nor forget his birthright as a soldier of the Empire. (ENT: "The Aenar")
In November 2154, he oversaw a plan to use a drone-ship – capable of disguising itself holographically – to destabilize relations between the Andorians, the Tellarites, and many other species in a certain region of space. The project was backed by Senator Vrax.
The drone-ship was controlled from Romulus and piloted via telepresence by the Aenar Gareb, who had been abducted for this purpose. When the drone-ship was boarded by Commander Tucker and Lieutenant Reed of the Earth ship Enterprise, Valdore attempted to coerce them into complying by irradiating the vessel. Tucker was injured by the radiation, but Reed sabotaged the drone-ship by setting a phase pistol on overload on its bridge. The resulting explosion significantly damaged the drone-ship. Subsequently, Tucker and Reed were rescued by Enterprise. As the drone-ship plan began to unravel, Vrax threatened Valdore with his Reman bodyguards.
Later, Valdore ordered Gareb to control two drone-ships simultaneously. When Gareb was telepathically contacted by his sister Jhamel, he disobeyed his Romulan captors and used one drone-ship to destroy the other. In response, Valdore shot Gareb in the neck with a disruptor pistol.
Valdore's project was a complete failure because its result was exactly opposite of that which was intended. Under the leadership of Captain Jonathan Archer, the Tellarites, Andorians, Humans, and Vulcans came together to locate and destroy the drone-ship. This temporary alliance sowed the seeds for the formation of the United Federation of Planets. (ENT: "Babel One", "United")
Valdore was played by veteran Star Trek performer Brian Thompson. He "loved" the character's costume and make-up but did not find the wig for Valdore's hair easy to wear. (Star Trek Magazine issue 123, p. 95)
In the Pocket ENT novel The Good That Men Do Valdore was released from prison when the Romulans became concerned over the creation of the Coalition of Planets and decide that Valdore's strategic abilities are needed to disrupt the ceremony. The implication was that he was in charge of the Romulan military during the Earth-Romulan War. In 2160, after the defeat of the Romulans at the Battle of Cheron and shortly thereafter during the negotiations that eventually established of the Neutral Zone, Valdore planned on using the defined border and buffer zone to rebuild the Romulan military to the point that they could successfully attack Earth and its allies. Valdore was assassinated by First Consul T'Leikha because of the Romulan losses sustained in the war and for any 'future embarrassment' for the Romulan Star Empire. She was in turn swiftly executed by Valdore's aide. (Pocket ENT novel To Brave the Storm)