Dennis O'Neil began his career as a comic-book writer in 1965 at Charlton, where then-editor Dick Giordano assigned him to several features. When Giordano moved to DC, O'Neil soon followed. At DC, O'Neil scripted several series for Giordano and Julius Schwartz, quickly becoming one of the most respected writers in comics.

O'Neil earned a reputation for being able to "revamp" such characters as Superman, Green Lantern, Captain Marvel - and Batman, whom O'Neil (with the help of Neal Adams and Giordano) brought back to his roots as a dark, mysterious, gothic avenger.

Besides being the most important Batman writer of the 1970s, O'Neil served as an editor at both Marvel and DC. After a long tenure as Group Editor of the Batman line of titles, he retired to write full-time.