He's best known for creating the classic crime character Modesty Blaise whith artist Jim Holdaway. The continuing thriller strips originally appeared in London newspaper, the Evening Standard in 1963 and ran for almost forty years.
Adventurer, spy, smuggler and seductress, fans including the likes of Quentin Tarantino, Modesty Blaise was dubbed the high priestess of pulp, providing crime thriller storylines with sass, wit and a touch of glamour. Films were made starruing the character in the sixties, eighties and noughties and O'Donnell wrote a number of novels as well as continuing the strip until 2001. At this point he requested that no more Modesty Blaise stories be written by anyone.
Titan Books recently began bringing the Modesty Blaise strip back into print, with sixteen collections of the classic strip and four more scheduled over the next year.
O'Donnell also wrote a number of gothic romance and adventure novels under the name Madeleine Brent.