Doctor M'Benga was a male Starfleet medical officer who served aboard the USS Enterprise under Captain James T. Kirk, during a five-year mission the starship undertook. In the event of Chief Medical Officer Leonard McCoy's absence, M'Benga became the ranking CMO.
He conducted his medical internship on Vulcan, an experience that made him particularly skilled in treating members of that species. In 2268, M'Benga treated First Officer Spock for a serious gunshot wound. He had to slap Spock violently to bring him out of his Vulcan healing state. (TOS: "A Private Little War")
Later that year, Dr. M'Benga supervised Dr. Sanchez, a colleague of his, with an autopsy of transporter operator Ensign Wyatt, who had died after the Losira merely touched him. M'Benga reported that Wyatt's entire body had suffered cellular disruption: the touch had "blasted" every cell in his body from the inside out. In reply to Spock's inquiry regarding the incident, M'Benga explained that his fellow clinician, Sanchez, had found that no known disease-causing organism had been responsible for the death. He later reported that Engineering Technician John B. Watkins had also died from a similar cellular disruption. (TOS: "That Which Survives")
Background information Edit
Doctor M'Benga was played by Booker Bradshaw.
This character was actually created by Darlene Hartman, in her purchased (but never produced) TOS script entitled "Shol". The notes for that script give Dr. M'Benga's first name as "Joseph", and note that he was originally a native of Uganda. His brother, Commander Simon M'Benga, was the first officer of USS Hope, a Federation hospital ship, in the planned-but-never-developed Star Trek spinoff Hopeship. 
An early draft of the script for "Obsession" (dated 6 September 1967) included a character by the name of "Nurse M'Benga". However, in research notes submitted for that episode (on 29 September 1966), de Forest Research noted, "M'Benga is the name of a doctor in 'A Private Little War'."
In a later inter-department communication between Gene Roddenberry and John Meredyth Lucas (dated 2 October 1967), Roddenberry noted that regarding "Nurse M'Benga. There is no such Nurse existing on our spaceship. If we need a Nurse, we use our semi-regular Nurse Chapel."
The first draft script for "That Which Survives" misspelled this character's name "Mboya".
Jean Lorrah's novel The IDIC Epidemic gave his first name as "Geoffrey". Meanwhile, David Mack's Harbinger established his first name as "Jabilo", a word for "healer" in Kenyan according to the author,  and set his assignment before the Enterprise as Starbase 47, also known as "Vanguard".
The Vulcan Academy Murders presented M'Benga's time at the Vulcan Academy hospital and McCoy's subsequent recruitment of M'Benga for Enterprise. Tying into this, The Klingon Gambit, by Robert E. Vardeman, established that his internship on Vulcan lasted four years.
M'Benga's mirror universe counterpart appeared in the novel The Sorrows of Empire. Following Leonard McCoy's death from xenopolycythemia in 2269, he succeeded him as the chief medical officer of the ISS Enterprise. He continued to serve in that position until at least 2287. The novel likewise stated that his first name was "Jabilo".