He was injured during a fight with the Klingons, when they attacked the Enterprise in 2268. Later, he was healed by the Beta XII-A entity because the entity needed his violent thoughts. Like the rest of the crew, he was manipulated by the alien entity. (TOS: "Day of the Dove")
Background information Edit
Johnson was portrayed by David L. Ross.
There were plans to rename this character Galloway, the name of a similar recurring character that Ross usually played in Star Trek: The Original Series; a note written on 20 August 1968 noted the name change, pointing out that references to "Johnson" in the revised final draft of the script were to be replaced with "Galloway".