John Leeson voiced K9 on Doctor Who from 1977 to 1979, beginning with The Invisible Enemy through The Armageddon Factor and again in 1980–1981, from The Leisure Hive and ending with Warriors' Gate. He was also seen as the character Dugeen during the serial The Power of Kroll.
Tom Baker detested the character of K9, but got on very well with Leeson. A favourite joke during filming breaks was for Baker to work out crossword puzzles, asking the K9 prop the clues with Leeson giving the answers in character. The short sketch Merry Christmas Doctor Who involved K9 telling the Fourth Doctor and Romana I that for Christmas he would like "enough ball bearings to last the series through and fewer sill irons on the floors of the planets we visit," as well as the robot dog downing some gin before beginning a drunken rendition of "We Wish You a Merry Christmas." When Leeson appeared on Tom Baker's edition of This Is Your Life in 2000, Leeson entered on all fours.
Leeson himself became frustrated with the character. He left the programme after the conclusion of the 16th season in 1978. David Brierley took over for four stories (one of which was never completed due to a BBC strike) the following year. Sadly, his vocal performance was no match for Leeson's. When John Nathan-Turner took over as series producer for season 18 in 1980, he tempted Leeson back, with the promise the character would be "killed off" that year. Although the character was, eventually, not killed off, merely written out, Leeson agreed to return.
Leeson also agreed to return to voice K9 in K9 and Company (1981), a spin-off from Doctor Who which was not picked up after its pilot episode was aired. He also voiced K9 for the twentieth anniversary episode of Doctor Who, The Five Doctors, though it was little more than a cameo appearance. He has also reprised K9's voice in two BBV and several Big Finish audio dramas including Zagreus and the Gallifrey series, as well as appearing in the Reeltime Pictures spin-off video Downtime as Anthony the DJ. He also voiced K9 for its return in the 2006 Doctor Who episode School Reunion and then again in Journey's End. Leeson voiced K9 during all of the robot dog's appearances on The Sarah Jane Adventures. Leeson's vocal contributions to Doctor Who can also be heard as other characters in the serials The Invisible Enemy and the twenty-fifth anniversary story Remembrance of the Daleks in 1988.
Other credits include appearances in Blake's 7 (season B episode "Gambit"), Jigsaw in which he played the giant Biggum, who was so tall only his feet were seen and his voice heard!) , Sorry!, 'Allo 'Allo! and Bungle in the children's television series Rainbow. John is also the voice behind the Space Gypsy characters on Children's UK hospital radio.
Today, he is now most likely to be heard as continuity announcer on Channel 4 in Britain. His rich, plummy tones show how excellent his performance as K9 was, as it is totally unlike his own voice. In 2009 and 2010, he voiced the character in the K9 TV series.