Dr. John Lawn, usually called "Mossy", is probably the only medical doctor in Ankh-Morpork actually known to cure or heal anyone. Normally, doctors are more avoided than the plague: the plague may not kill you, and certainly won't charge you for it. Lawn credits his training in Klatch, where patients were expected to get better where he learned techniques other Morporkian surgeons distrusted, but which kept patients alive for longer than it took to pay the bill. He also gave free treatment to those who needed it, including those who had been tortured by the Cable Street Particulars. While he is unlikely to give his patients a little trim or a shave when performing surgery (barbers traditionally working as surgeons in the early days of medicine in the Roundworld, he makes use of a number of curious and often alarming implements in his line of work, not the least of which are used for delivering babies. Being outside the medical establishment, he has worked for many years in a rough area near The Shades as a "pox doctor," with many patients among the Seamstresses.
There, he met and treated John Keel/Sam Vimes when Sam is transported back in time in the pursuit of the psychopathic killer Carcerin Night Watch. He lets Sam sleep in his spare room and serves as the surgeon for the short-lived army of the Glorious Revolution. When Sam Vimes returns to the present time in Night Watch, he finds his wife in life-threatening labor with their first child. Vimes literally flew to Twinkle Street to get the now-thirty-years-older doctor, and rewarded his success (and his silence as to the real identity of "John Keel") with "one-hundred thousand dollars and the freehold of a large corner site in Goose Gate" near the old city wall. By the time of the events of Going Postal, the sprawling building had become the Lady Sybil Free Hospital.
Mossy continues as Chief Of Medicine there, apparently still fighting off the non-professional attentions of the nursing staff despite advancing middle age. Dr Lawn's preferred method of dealing with the nursing staff is to throw a handful of chocolates in one direction and run in the other as fast as possible, a play on the concept that chocolates are better than sex. Mossy serves as the family doctor to the Vimes family. Lady Sybil mentions that Dr. Lawn has told Sam to eat more fruit and vegetables - tomato ketchup not counting as such - and to stay away from eating greasy food. It is doubtful that Sam takes his advice however.
Later books state that the hospital is also used by Dr. Lawn to train new doctors in the same techniques that he had learned as a young doctor in Klatch. He ialso employs Igorinas who wish to practice their full range of medical skills and not just assist their Igor counterparts. Tiffany Aching's friend Preston is one of his med students, now a surgeon in his own right (The Shepherd's Crown).
Dr. Lawn's personality can be described as quiet (if a tad sarcastic) and almost unshockable. During the events of the Glorious Revolution Lawn comments to John Keel/Sam Vimes that, when he dies, he wants a bell left on his gravestone so he can have the pleasure of not getting up when people ring.
Lawn is quite possibly one of only two people (besides Vimes) who know the truth concerning John Keel: the other is the Patrician.
The origin of his name needs no explanation. The character of Dr. Lawn is based on an actual retired GP of the same name based in West Yorkshire.