He was born in Whitechapel to an unwed mother who died young, leaving him in the care of his "auntie"—an aged and alcoholic prostitute—who "taught him how to duck."
He served in the French Foreign Legion, including the gruesome Battle of Dien Bien Phu, and later lived in Thailand, where he learned his skills as a Thai boxer.
It is during his time as a Thai boxer in Saigon (or Bangkok, O'Donnell changed the story at some point) that he first meets Modesty Blaise, who despite being only twenty, runs a powerful (but moralistic) criminal organisation called The Network.
from London called The Treadmill—and when Sir Gerald Tarrant offers Modesty a chance to put her skills to work on behalf of British Intelligence, she easily recruits Willie to fight by her side.
Very sociable, he has a number of female acquaintances from all social levels (as Modesty humorously describes them, "from premier cru to honest vin de pays"), and is often reminded of anecdotes of them in the most unusual situations (such as when swimming in shark-infested waters or when hauling heavy objects up slopes).
Like Modesty, he is a polymath of the first order: adept in safe-cracking and lock-picking, micro-electronics, expert at military tactics and a student of the criminal arts.
He speaks several languages, including German, Spanish, Arabic and French; his English has a heavy cockney tone to it, though he is able to turn it on and off like a switch.
He does not like pistols (because he thinks that guns give people too much confidence), and he and Modesty have also stated that he is a poor shot, but he has shown himself to be capable on the rare occasions that he uses such weapons.
During a caper to protect a trainload of innocent people, including some children, from a group of rebels, Modesty suddenly kisses Willie romantically (for the first and only time) and reveals to him that she is dying of a brain tumor.
Terence Stamp portrayed Willie Garvin in the 1966 film version of Modesty Blaise, a year after the publication of the first novel.
Later, the writers of the film choose to break O'Donnell's anti-romance taboo and have Willie and Modesty fall in love and even talk about getting married.
Also in the early 1980s, John Thaw recorded an audio adaptation of I Had a Date with Lady Janet which he performed in character as Willie Garvin.
One of the Modesty Blaise novels, Last Day in Limbo, was adapted as a BBC World Service radio drama in 1978 with James Bolam as Willie.