Shortly after being joined in the 2280s, Curzon became a member of the diplomatic corps of the Terran Empire and quickly gained a reputation as one of its diplomats who truly understood the Klingon mindset. He attributed this to the fact that he did not let the Klingons intimidate him. While other diplomats attempted to appease them, he challenged them. Although he was only in his twenties at the time, Emperor Spock appointed him as the ambassador to Qo'noS in 2286.
Curzon was a well-known womaniser. Immediately before meeting Spock and being appointed ambassador, he presided over a court of his own in the throne room of the Kyoto Imperial Palace, flirting shamelessly with a bevy of beautiful young women, several of whom were the wives and concubines of senior imperial dignitaries. He cracked jokes and playfully touched the women's cheeks and chins as they laughed.
In 2287, Spock learned through his erstwhile protégée, Captain Saavik of the ISS Enterprise, that several Trill symbionts in the symbiont pools in the Caves of Mak'ala on Trill had become infested with a hostile parasitic intelligence, which he believed could ultimately pose a threat to not only his plans to dismantle the Terran Empire but all sentient life in the galaxy. In response, Spock ordered that all symbionts were to be exterminated and signed an order empowering the Imperial Starfleet to hunt down and execute all joined Trill with the exception of Curzon. He spared Curzon's life as a reminder of what he had done and what he had become.
In his capacity as the imperial ambassador to Qo'noS, he attended a conference held by political and military elite the Klingon Empire on Korvat in 2289 to celebrate their recent conquest of the planet, an unarmed farming colony less than a lightyear from Klingon space which had previously been located in inviolable Terran territory. During a speech by General Kang, Curzon arrived late and made no effort to remain inconspicuous. Within minutes, he spat at Kang's feet in disgust and decried the manner in which the Klingons were celebrating the "least audacious victory in [their] history" and accused them of having a "miserable empire" before storming out, which earned him Kang's wrath. Having observed this incident, Regent Gorkon and his senior military adviser General Chang speculated as to whether this was a political stunt or whether Spock had chosen the young and inexperienced Dax in haste. Gorkon favoured the former explanation as he believed that the latter did not seem to be consistent with what they knew of Spock or Curzon's reputation.
In 2291, he met with Spock in the Imperial Palace for the final time. At this meeting, Spock demanded his resignation and ordered him to go into seclusion as part of Memory Omega on Regula 1. Curzon confessed that he was aware that Spock was responsible for the extermination of all other joined Trill while Spock revealed to Curzon why he had spared his life.
Before departing, Curzon reminded the Emperor of the example of the ancient Terran dictator Julius Caesar. When he returned to Rome from his wars of conquest, he kept a slave at his back to whisper in his ear, Sic transit gloria mundi: "Thus passes the glory of this world." Spock told Curzon that he was aware of the lessons of history and that his life would forever be the whisper in his ear. (TOS - Mirror Universe novel: The Sorrows of Empire)
Curzon spent the rest of his life on Regula 1. Shortly before his death in January 2379, he met with Ezri Tigan, a young Trill woman and a member of the recently victorious Terran Rebellion, and convinced her to become the eighth host of the Dax symbiont in order to prevent the species from going extinct. Ezri Dax then assumed command of the Galactic Commonwealth vessel Defiant. (ST - Mirror Universe novel: Rise Like Lions)