After returning to Britain he had a desk job with the B.U.P., where he remained until, in 1948, Clark resolved to earn a living through journalism and devote his best energies to the writing of his books.
He wrote sixty-six books, covering subjects ranging from mountain climbing (over a dozen titles), the atomic bomb, Balmoral Castle, and world explorers, as well as novels of alternate history.
He also wrote a number of biographies of a diverse range of historical figures, including: Charles Darwin, Thomas Edison, Albert Einstein, Benjamin Franklin, Sigmund Freud, J.
After leaving school for his first job, and having joined an amateur theatrical group, Clark met Irené Tapp (1901-1977), fifteen years his senior.
She too contributed to her husband's writing career with "all the skills of an executive secretary" and acting as a valuable researcher for Clark.