Early in Valdore's career, he served as a senator in the Imperial Romulan Senate, until he challenged the Empire's precept of unlimited expansion across the galaxy. Because of this, he was expelled from the Senate and was facing execution before fellow Senator and old friend Vrax intervened with the First Consul to have the Praetor spare Valdore's life. Following his expulsion from the Senate, Valdore vowed never again to forget his birthright or duties as a soldier of the Romulan Star Empire. (ENT episode: "The Aenar", ENT novel: The Good That Men Do)
In late 2154, Valdore hatched a plan to destabilize relations between the Andorians and the Tellarites, by using an Aenar to telepathically control a drone ship and attack either side to sow distrust. However, that plan was defeated by Captain Jonathan Archer and the crew of the Enterprise (NX-01). (ENT episodes: "Babel One", "United")
Following the loss of the prototype drone ships, Valdore and Senator Vrax were both stripped of rank and imprisoned. Several months later, the First Consul would free Valdore from confinement and restore him to the rank of Admiral. He was also successful in convincing the First Consul to release his friend Vrax and to not have him executed.
Valdore then aided the Empire in preventing the Coalition of Planets from gaining an advantage over the Star Empire. This was achieved by having a Romulan ship ram Coridan III, causing a massive firestorm that killed nearly half a billion people. While the attack did deny the Coalition the much needed Coridan dilithium, it also had the effect of strengthening the resolve of the remaining Coalition members to oppose a Romulan invasion of their space. Valdore felt some guilt over what had happened, feeling that the plan had not been worthy of a military man, and was instead wholesale slaughter of innocent people. (ENT novel: The Good That Men Do)
In 2155, he was involved in a new plan for Romulan expansion which involved the use of the newly developed arrenhe'hwiua telecapture technology allowing the Star Empire to remotely take over enemy vessels. Using the captured ships, Valdore attempted to create tensions between the Coalition of Planets and the Klingon Empire in order to bring about a war between the two nations which would ultimately weaken them allowing the Romulans to claim the spoils. He was also involved in using the captured Coalition ships to attack member races so that mistrust would emerge between the various races and shatter the alliance. Though this did not happen, the Romulans were successful in getting a beachhead on Calder II which would serve as a means to spearhead an invasion of the Coalition homeworlds. (ENT novel: Kobayashi Maru)
Valdore was seriously injured in the Battle of Cheron, which was won by the United Earth Starfleet and their allies. Soon after the battle the Earth-Romulan War came to an end as the Empire negotiated the Treaty of Alpha Trianguli with Earth and its allies. This treaty ended the war and established the Romulan Neutral Zone. Both Praetor Karzan and Valdore supported the idea of the Neutral Zone as a defined border with a buffer zone would allow the Empire to rebuild its forces.
The First Consul T'Leikha was furious over the new treaty, feeling that the Empire was showing weakness to Earth and other interstellar powers. A few weeks after the end of the war, she confronted Admiral Valdore in his office over her objections to the treaty, and assassinated the Admiral. T'Leikha was herself executed a few moments later by Valdore's aide Terix Val'Danadex Trel tr'Llweii. (ENT - The Romulan War novel: To Brave the Storm)