Peter O'Donnell complains that of his original screenplay, only one line remains: "What do you know about Wilberforce?"
When Willie Garvin is introduced, there is a close-up of a painting of the comic strip version of Modesty Blaise.
At one point Modesty Blaise is shown looking at copies of the real-life Modesty Blaise comic strip, which originally appeared in the London Evening Standard.
Peter O'Donnell was invited to write a novelization of this film. The book, based upon O'Donnell's original screenplay rather than this film, was a huge success and spawned a series of best-selling novels that O'Donnell wrote until 1996.
Clive Revill was fourth choice and the part was not cast until shooting had begun.
The theme tune is performed by an uncredited David & Jonathan.
When the shorter red haired assassin puts out a cigarette in an ashtray, the "Dutch" logo on the ashtray spells: Fokking.
When Modesty rings the doorbell an an apartment building, the second name down on the address sign next to the door bell says in mock "Dutch": Van Dijk.