He can, however, also be quite clumsy[3] and unsubtle, once leading the Doctor to declare, in a moment of frustration and at the top of his lungs, that "Harry Sullivan is an imbecile!"

The character was originally devised by the production team as a means of handling any action scenes required in episodes when they had envisioned that the new Doctor would be played by an older actor, as had previously been the case (Sarah Jane even jokingly compares Harry to James Bond at one point).

At the conclusion of this story he chooses to return to London by train rather than by TARDIS with the Doctor and Sarah Jane, who continue their adventures without him.

[1] A later production team gave some consideration to bringing Harry Sullivan back for a guest appearance in the 1983 story Mawdryn Undead, part of the programme's twentieth anniversary season.

[2] After leaving the cast of the programme, Ian Marter went on to pen several novelisations of Doctor Who stories for Target Books, writing an original novel, Harry Sullivan's War, for them in 1985.

The New Adventure Blood Heat (1993) by Jim Mortimore briefly depicts a parallel universe version of Harry serving on a nuclear submarine in a dystopian world ruled by the Silurians, where he manages to save the life of the Seventh Doctor's companion Bernice Summerfield.

In David A. McIntee's Past Doctor Adventure The Face of the Enemy (1998) Harry is seen still working for the Royal Navy before his secondment to UNIT, which he first encounters in the novel.

He also appears in the novel Wolfsbane by Jacqueline Rayner, where he briefly aids the currently-amnesic Eighth Doctor in dealing with a plot involving a woman who believes herself to be the reincarnation of Morgan Le Fay in 1936 when the Fourth Doctor's TARDIS accidentally materialises there long enough for Harry to leave, having apparently been drawn off course by its damaged future self.

In the Big Finish Productions audio drama UNIT: The Wasting (2005), Commodore Sullivan (who is working with NATO) is called on by the Brigadier for a favour but does not have a speaking part.

Harry's previously unknown younger stepbrother, Will Sullivan (also a medical doctor), appears in the second series of the Sarah Jane Smith Adventures (2005–2006) audio plays by Big Finish, voiced by Tom Chadbon.

In December 2018 the free subscriber short story Tuesday[7] by Tony Jones also featured Harry Sullivan and was narrated by Stephen Critchlow.

In 2019, Big Finish released their third story featuring the character of Harry Sullivan narrated by Leela (Louise Jameson) titled #HarrySullivan.