As a pulp science fiction magazine writer, he was published in Galaxy and Worlds of if. Popular in the 1960s, his stories were often set in utopian societies and had reformist themes. He was the first to write an original novel based on the TOS TV show that came out in hard cover and written for a young adult audience. He wrote under numerous pen names, some of them variations of his birthname, others like Todd Harding, Mark Mallory and Bob Belmont, completely different. A compilation of his work, The Best of Mack Reynolds, was published in 1976.