O'Donnell begun to write professionally prior to World War Two at the age of 16. From 1938 and during the war he served as a NCO in mobile radio detachment of Royal Signal Corps in 9th army in Persia in 1942. Afterwards his unit was moved to Syria, Egypt, the Western Desert, Italy, and Greece in October 1944.
After the war O'Donnell begun to script comic strips. 1953-1966 he wrote for Garth and 1956-1962 Romeo Brown (with Jim Holdaway as an artist). He also wrote for women's magazines, children's papers and an occasional novel.
His most famous creation, Modesty Blaise, was first published in 1963.
O'Donnell has also written romance books and television (Take a Pair of Private Eyes) and movie (Revenge of She) scripts. His other famous books are romances he has written under the pseudonym Madeleine Brent.
Books written under the name Madeleine Brent: