Johnson dropped out of school in ninth grade, spending time in the U.S. Army and college before breaking into writing with an episode of Alfred Hitchcock Presents in 1959. In 1967, his script "The Man Trap" became the first aired episode of the original Star Trek, which saw the Enterprise crew killing a shape-shifting parasite. While not considered the first official episode, it's said to have provided the tone and themes for the show's future. He also contributed to episodes of Route 66, The Twilight Zone, The Law and Mr. Jones and Kung Fu.
Johnson was perhaps best known for Logan's Run, a novel about a future world in which people were killed at age 21, which was adapted into a 1976 film starring Michael York. Later in life, Johnson became a marijuana legalization activist.
Johnson lived in Los Angeles and is survived by his wife Lola, daughter Judy Olive and son Paul.