He was taken into the Guild as a child because the administration took pity on him after he lost his parents at a young age. It is never specified what happened to young Jonathan's parents; it is, however, implied that what happened to them was young Jonathan.

He is boyishly handsome, with curly hair and a ready smile, but these features are ruined by his eyes. One is glass and the other is off-white, with a tiny, pinhole-sized pupil. He is, undoubtedly, a genius, but also sociopathic to an astonishing degree. His mind has been compared at times to both a corkscrew and a shattered mirror (i.e. something brilliant, sharp, and dazzling, but also fundamentally and irrevocably broken). His main problem seems to be that he 'sees things differently from other people, in that he sees other people as things'. This gets him into a spot of trouble early on, as his assassinations often include the brutal murders of not just the target, but also their family, their servants, and very often their pets, all of which is a direct violation of Guild protocol.

However, when the Auditors of Reality need someone to assassinate the Hogfather, Mr. Teatime is just the man for the job. When questioned, he reveals to his superior, Lord Downey, that he often used to lie awake in bed at night and think of ways to kill, not only the Hogfather, but also the Tooth Fairy and the Soul Cake Tuesday Duck (Discworld's version of the Easter Bunny), among others (including Death...). He commissions the help of a cadre of lowlifes, ranging from big Banjo Lilywhite to the locksmith Mr. Brown, and sets about putting his ingenious plan into action, which inevitably pits him against Susan Sto Helit and her grandfather, Death.

Most of the time, Mr. Teatime seems a pleasant, albeit odd young man. He only seems to get irritated when people mispronounce his name. However, he possesses physical abilities which defy physics, and has been known to perform feats such as stabbing through all layers of clothing but stopping before hitting flesh, doing the same thing but with eyelids instead of clothing and eyes instead of flesh, flipping on thin air, and killing so fast he appears to be a blur, all of which he will do without any notice or provocation. Rumours among his associates (somewhat confirmed in the non-canonical GURPS Discworld sourcebook) imply that the glass eye is in fact a scrying crystal, which might go some way towards explaining his abilities but also means that he implanted notoriously erratic Discworld magic into his own eye-socket.

The Assassins' Guild lists him as having vanished without trace since the events in Hogfather, though they have named a "Teatime Prize" in his honour, which is given annually to the student who devises the most creative, but technically accurate, hypothetical inhumation, preferably complete with maps and a proposed route that avoids or disables any guards.

In the Sky One adaptation of Hogfather, Mr. Teatime is played by Marc Warren. Warren plays Teatime with a soft American accent, partly based on Johnny Depp's version of Willy Wonka (another unpredictable and "childlike" character). His homicidal character is shown through the way he interacts with his victims, for example, Teatime has just entered a heavily locked door, upon entering he sees a cowering guard, he tells the guard "I am your worst nightmare" the guard stop's to consider, then asks "Is it the one with the big whirring thing and the teeth..." upon which Teatime replies softly "Sorry not that one. I'm the one where this stranger comes out of nowhere, and kills you" Teatime then proceeds to stab the guard, along with a dry remark.

Mr. Teatime is very popular within the Pratchett fandom. This is most likely due to his interesting personality, and he is usually paired with Susan in fanfiction.[citation needed]