He joined the Doctor and Leela (DW: The Invisible Enemy), but parted company with the Doctor when he decided to stay with Leela who had choosen to remain on Gallifrey with her new lover, Andred. (DW: The Invasion of Time) Later, it is believed that the Time Lords assigned K-9 the command of a space vessel called K-NEL and gave him occasional assignments. (The Adventures of K9)
K9 Mark 1 is rumoured to return in the first episode of K-9 (TV series). Bob Baker has said that the K9 model that appears in the early part of the first episode is "identical to the one Leela had in Galifrey [sic]". This means K9 Mark 1 might have survived the Time War. In the pilot episode, he will appear as last seen in Doctor Who but will before expiring, insist his companion initiate his "regeneration" sequence, from which a new and more futuristic form of K-9 Mark I will emerge.