Leeson trained at Royal Academy of Dramatic Art and his varied stage and television career spans half a century.

Leeson's vocal contributions to Doctor Who can also be heard as other characters in the serials The Invisible Enemy and Remembrance of the Daleks.

Leeson played the role of "Prosecutor 2" in the 2005 stage production of The Trial of Davros and has compiled a one-man show, A Dog's Life, directed by Richard Fawkes.

In November 2013, Leeson appeared as K9 in the one-off 50th anniversary comedy homage The Five(ish) Doctors Reboot.

His radio work includes the comedy serial All That Jazz (1990) in which he played the character of Mike[6] and additionally provided the scripted continuity link at the beginning.

In 2010 John appeared in a character role in the independent film Rebels Without A Clue with actor Rik Barnett.

[8] Away from stage and screen he was regularly heard as continuity announcer on Channel 4 when his warm tones proved a distinct contrast to the K9 voice.

[10] It was superseded by an updated and expanded version entitled Tweaking the Tail which was published in July 2013 by Fantom Films.