Yeoman J.M. Colt was a female Starfleet enlisted crewman who lived during the mid-23rd century. She served in the operations division aboard the USS Enterprise under Captain Christopher Pike. In 2254, she served as the captain's personal yeoman, replacing his previous yeoman, killed in a fight on Rigel VII.
When Pike was captured by the Talosians, Colt was one of six crewmembers who volunteered to beam into the Talosians' underground city to rescue him. In fact, the Talosians had read Colt's attraction to Pike and manipulated events so that only she and Una Chin-Riley transported down. All three Enterprise crewmembers escaped the Talosians alive and uninjured. (TOS: "The Cage")
Footage of her experience aboard the Enterprise under Captain Pike during the original visit to Talos IV, from 13 years prior, was transmitted from that planet during Spock's fictional court martial aboard the same ship in 2267. (TOS: "The Menagerie, Part I", "The Menagerie, Part II")
Yeoman Colt was played by Laurel Goodwin in "The Cage". Cindy Robbins was considered among the finalists for the role of Colt, even going so far as to appear in makeup test shots with Leonard Nimoy, while wearing a prototype Starfleet uniform.
Colt's name was not spoken onscreen, and was derived purely from script or background sources, with the following character biography appeared in the series proposal Star Trek is...:
The Captain's Yeoman. Except for problems in naval parlance, J. M. Colt would be called a yeo-woman. With a strip-queen figure even a uniform cannot hide, Colt serves as Captain's secretary, reporter, bookkeeper – and with surprising efficiency. She undoubtedly dreams of serving Robert April with equal efficiency in personal departments.
The note about Colt's title not being yeo-woman was included in the scripts of "The Cage", which went on to say of the character, "About twenty, she's pert and shapely, but carries herself with trained precision." Her assignment as the captain's yeoman was referred to in the scripts as being merely temporary, though this line of dialogue is not in the final version of the episode. In another example of scripted but ultimately omitted dialogue, Colt was established as having had the same training as everyone else on the Enterprise, despite being new to the ship herself. 
The comic book series, Star Trek: Early Voyages, named her "Mia Colt". Although this might seem contradictory to her first initial being "J", her second initial was "M", and she may have gone by her middle name rather than her first.