K9 Mark I was created about the year 5000 by Professor Frederick Marius, who gave the dog to the Fourth Doctor and Leela. He was left behind with Leela on Gallifrey. (TV: The Invasion of Time) Some time later, he was blown up by a bomb on Gallifrey, (AUDIO: Imperiatrix) but survived. This model came to an end when Alistair Gryffen's Space-Time Manipulator randomly pulled him to London in the year 2050. There, he almost immediately self-destructed to protect Gryffen and a group of teenagers from Jixen warriors. (TV: Regeneration)
Later he defeated Ahab, who tried to convince him he had murdered Zanthus Pia. He became part of the K9 Unit, being made the responsibility of Gryffen and under the control of Department inspector June Turner. (TV: The Bounty Hunter) He foiled Inspector Drake's plan to control the population using Cerulium (TV: Sirens of Ceres) and experienced fear. (TV: Fear Itself) He aided in the defeat of echoes of Gryffen's family, (TV: The Fall of the House of Gryffen) Orthrus, a duplicate of him created by Drake to frame him (TV: Jaws of Orthrus) and the Bodach. (TV: Dream-Eaters) He met Nehetka and Geb of the Anubians, a race his previous incarnation liberated. He was so obsessed with regaining his lost memories that he didn't notice Gryffen, Starkey and Jorjie's takeover by the Anubians. He soon found out when the Anubians attacked the Department. With Darius' help, he defeated them. (TV: Curse of Anubis)
He did not use contractions such as "you've" for "you have" and addressed others by titles such as "Master", "Mistress", "Doctor-Master" (to refer to the Doctor) or "Young Master" (Adric, Starkey and in one instance, Clyde). Though he did not seem to resent his subordinate status, he sided with the Doctor's companions over the Doctor and showed a dark side, regarding other artificial intelligences with contempt. (TV: The Invasion of Time, TV: The Wedding of Sarah Jane Smith, TV: Robot Gladiators) He relished a brief chance to act as a figure of power. (PROSE: The Well-Mannered War)
Jealous, Possessive by Paul Magrs, the "Scorpio" story in Big Finish's anthology Short Trips: Zodiac features K9 Mark I and Mark II relaying their exploits to each other, and their veiled put-downs to each other reveal that each considers the other to be the "inferior" version. This attitude is also occasionally hinted at in the way the two units refer to each other in the Gallifrey audio series.
With his great intelligence, he had a tendency to bore people with facts and did not stop immediately when asked, as he did to Leela on Pluto. When being told that he would have to stay behind in the TARDIS, K9 often argued, giving the Doctor reasons why he should go such as "he would be a good dog." K9 always wanted to assist the Doctor. He had great intelligence and skill at chess, claiming to be able to beat the Doctor in six moves (although this may only have been because he was programmed with all chess games since 1886). (TV: The Sun Makers, The Androids of Tara) K9 had an aggression mode. (TV: State of Decay)
Although John Leeson has been the primary voice actor for K9 since his introduction, two other actors have voiced the character. David Brierley voiced K9 during Season 17 after Leeson chose not to return to the series (Leeson subsequently signed up again for Season 18), and Roy Skelton, better known for his work voicing Daleks, voiced the character in Destiny of the Daleks when all was required of K9 was a croaking sound due to the character contracting a form of robot laryngitis.
In 1985, a series of gamebooks featuring the Sixth Doctor were published by Severn House under the title Make your own adventure with Doctor Who in Britain and Find Your Fate — Doctor Who in the United States. These books were actually written by scriptwriters for the television series. Martin wrote Search for the Doctor which takes place in the mid-21st century and features K9 Mark IV being reunited with the Sixth Doctor long after Sarah's death.