Leonard is a man of indeterminate age (he is described as "one of those people who start looking venerable at thirty, and proceed to look that when they got to ninety"). Originally from Quirm, he moved to Ankh-Morpork some time ago and is currently a permanent "guest" of the Patrician. Before this he was a member of nearly every craftsman's Guild in the city, most commonly being kicked out for getting impossibly high marks on examinations or correcting the questions.
Like his counterpart, he is a great painter, being known for such paintings as Woman Holding Ferret and the Mona Ogg. (Leonardo's Lady with Ermine and Mona Lisa.) Also like his counterpart, he is left-handed and writes things backwards (ie. "ENNOGEHT"—"The Gonne"—in Men At Arms).
He is also a brilliant engineer. Like the early Leonardo he has designed a great number of war machines, and like the later Leonardo he holds war to be the worst of human activities. It has been noted, however, that the sketches which he makes of the devices he imagines variously show ". . . numbered parts and a list of instructions . . . " or ". . . practically include glue and transfers". Still others have noted that the sketches deal with the swift removal of inconveniently placed mountains, or show dying sailors leaping from burning ships into a boiling sea.
Despite this, Leonard views all his martial designs as intellectual exercises, and refuses to believe anyone would be mad enough to make them. His magnum opus in this department was an invention mentioned in The Fifth Elephant, which was described as a weapon of such destructive power, it would render war meaningless, as no one would ever dare use it. (a claim made by Alfred Nobel about dynamite; it was not fulfilled until the threat of mutual assured destruction by a nuclear holocaust much later) It is this naïveté that has led to his de facto imprisonment by the Patrician, lest someone other than him make use of the man's genius. But as long as he has paper to sketch on, and can watch the birds, Leonard barely notices his imprisonment. In fact, he has voluntarily returned to it, believing the world outside to be full of madmen. The hallway leading to his 'cell' is full of deadly, complicated traps, which he himself designed.
He has also designed many less dangerous devices, including a submarine (The Going Under The Water Safely Device), an Espresso machine (The Very Fast Coffee Machine) and a coding machine (the Engine for the Neutralizing of Information by the Generation of Miasmic Alphabets, see Enigma machine). His genius doesn't quite extend to names. He also finds it impossible to focus on anything. Or rather, he's focused on everything, all the time (see Attention Deficit Disorder). Many of his inventions started out as something completely different, before his mind shifted tracks half way through. (The Patrician often speculates on the fate of mankind should Leonard manage to keep his mind on one subject for any length of time.)
Again, like Leonardo, one of his greatest ambitions has been to build a working flying machine, something he achieves in The Last Hero.
It is worth noting that, although his name is Leonard "of Quirm", he is referred to as 'da Quirm' when his first name is unstated.