The primary voice actor for all versions of the character is John Leeson. He played K-9 Mk. I starting with "The Invisible Enemy" in Season 15 and continuing to the end of the season and "The Invasion of Time". Season 16 introduced K-9 Mk. II and Leeson continued to voice the character from "The Ribos Operation" to "The Armageddon Factor".
Leeson then chose to leave the series. A new actor, David Brierley, was cast to provide K-9 Mk. II's voice in Season 17, however early efforts to write out the character were already under way, and as such K-9 appears very little in the first few stories. K-9 makes only a brief appearance in "Destiny of the Daleks" and his entire vocal contribution is a cough; Roy Skelton, who was one of the Dalek voice actors, recorded the cough (technically making him the third K-9 actor). Brierley's first performance as K-9 was in "The Creature from the Pit", the third story of the season.
Brierley's final televised performance as K-9 was in the Season 17 finale story "The Horns of Nimon". An additional story, "Shada", was partially filmed and recorded, but was abandoned and never completed due to a strike. Brierley recorded his final work as K-9 for that story, though later restorations used John Leeson's voice.
For Season 18, which was determined to be K-9's final one on Doctor Who, Leeson was coaxed back, and he subsequently played K-9 Mk. II from "The Leisure Hive" through to "Warriors' Gate" when both he and Romana II were written out.
Later, the BBC attempted to launch a spin-off series titled K-9 & Company featuring Sarah Jane Smith and a new K-9, K-9 Mk. III. Leeson was again cast as K-9's voice, but only the pilot episode, "A Girl's Best Friend" was ever made. K-9 Mk. III later made a cameo appearance in "The Five Doctors".
When Elisabeth Sladen returned to Doctor Who in 2006's "School Reunion", John Leeson returned also as K-9 Mk. III, which was destroyed in the episode, but immediately replaced by K-9 Mk. IV. Leeson continued to voice this version of K-9 which appeared in a handful of episodes of The Sarah Jane Adventures and also "Journey's End". The character was not a regular on Sarah Jane due to the rights owner of K-9, Bob Baker, having arranged to feature him in a non-BBC spin-off series made in Australia titled K-9. This series aired for a single season over 2010-2011 and featured (here's where it gets confusing) the original K-9 Mk. I, who regenerates into a different K-9 Mk. II (usually referenced as Mark 2 to avoid confusion with the other) during the course of the pilot episode, "Regeneration". Leeson was cast to play K-9, but following the character's regeneration, he was allowed to play the role with more emotion in his voice. As of 2014, a second series of K-9 has officially been in "pre-production" mode since 2010.