Jocelyn Treadway (neé Jocelyn Darnell) was born on March 11, 2227 in Georgia, North America on planet Earth, not long after the birth of Leonard McCoy and not long before that of Clay Treadway. After Treadway's birth, the Darnells and Treadways loosely betrothed their offspring to each other.
By 2243, the two had become boyfriend and girlfriend, but Darnell quickly learned that Treadway was over-possessive and prone to extreme jealousy. When the two attended a high school social in 2244, Darnell openly grabbed the shy Leonard McCoy to dance with, angering Treadway. Treadway fought with McCoy, but was knocked to the floor by him, allowing the gentlemanly McCoy to leave with Darnell. This ended their relationship and was the beginning of that of Darnell and McCoy.
McCoy enrolled in Medical School in 2245, although he did his studying in his home in Georgia. McCoy and Darnell finally married in 2248 and had a daughter, Joanna McCoy around 2249. Treadway left Darnell's life (and presumably Georgia) shortly after her marriage.
The years between 2245 and 2253, when McCoy was studying voraciously, caused Darnell much loneliness, but it was in 2253 that Treadway reappeared in her life (in Georgia). Soon, the two were in love and committed infidelity, which McCoy discovered. This drove McCoy into depression. (TOS novel: Shadows on the Sun)
Soon after, McCoy saw an advert for practicing medicine in space, believing this was the best way to flee his current situation, McCoy signed up. Shortly after, however, Jocelyn left him and Joanna behind. With his new career about to begin, McCoy decided it would be best of Joanna lived with her aunt and uncle, Donna and Fred Withers, on Centaurus instead of residing on starships with him, which was impractical. (TOS novels: Planet of Judgment, Crisis on Centaurus)
A meeting of unspecified detail saw Darnell meet James T. Kirk in 2263, and the two would not see each other again until 2293. Although Kirk contacted Jocelyn via subspace in 2264, when he urgently needed to track down McCoy so that he could assume his post aboard the USS Enterprise, on the off chance that Jocelyn had heard anything. (TOS novels: Shadows on the Sun, Enterprise: The First Adventure)
In 2293, soon after establishing tentative peace with the Klingon Empire, the USS Enterprise-A was sent to planet Alpha Gederix IV (a.k.a. Ssan), along with the Diplomatic Corps team of Clay and Jocelyn Treadway. Assassins of Shil Andrachis – ironically, someone whose life McCoy had saved in his first visit to the planet – captured Jocelyn and Kirk. Although a rescue team was launched, Andrachis ultimately murdered Jocelyn, although Andrachis himself died in the event. Jocelyn's last words were to ask McCoy to dance with her, to which he promised that he would do so until the end of time. During the return flight to Earth, Clay commented to McCoy that, although Jocelyn might have been confused about what she had truly wanted, in the end McCoy was the only one of the two of them she had truly loved. (TOS novel: Shadows on the Sun)
- Though not confirmed, it is probable that Jocelyn was also the mother of Barbara McCoy.
In an alternate timeline in which John Frederick Paxton destroyed Starfleet Command and ended the talks for the Coalition of Planets in 2155, Jocelyn died a month after giving birth to Joanna in 2246, leaving Leonard McCoy to raise their daughter on his own. (TOS - Myriad Universes novel: A Less Perfect Union)
In the alternate reality created by Nero's temporal incursion, Leonard McCoy was divorced by 2255, but from a woman named Pamela Branch. (TOS movie, novelization & comic adaptation: Star Trek, TOS comic: "IDW Star Trek, Issue 17")