Joanna McCoy, the daughter of Leonard McCoy and Jocelyn McCoy, was born on Earth in 2249. Her early childhood was spent in the McCoy family home in Atlanta, until her parents separated and her mother left in 2253.
As her father was just beginning a successful career as a doctor in Starfleet, Leonard decided that he wanted Joanna raised on a pleasant planet instead of on starships, so he sent Joanna to live with her aunt Donna Withers and her husband, Fred Withers, on Centaurus. Leonard also purchased a house on Centaurus as a place where he could spend his leaves with Joanna. As time progressed, however, Joanna and her father drifted apart. (TOS novel: Crisis on Centaurus)
As Joanna grew older it was decided that she would pursue her education off planet and underwent schooling on Cerberus. In 2260, Cerberus suffered crop failure and the colonists faced famine. However, the colonists were saved when Carter Winston used his personal fortune to feed the colonists. (TAS episode: "The Survivor")
At the age of 18 (2266), Joanna was able to re-establish contact with her father, as she prepared to enter college to study to become a nurse and follow on her father's footsteps. As part of her training to become a nurse she worked at a hospital in New Athens. Following the detonation of an antimatter bomb at the New Athens Spaceport in 2269, Joanna had to treat the few survivors from the explosion which was quite harrowing for her. This horrible incident did give Joanna a chance to see her father when the USS Enterprise was called in to lend assistance. (TOS novels: The Better Man, Crisis on Centaurus)
In 2274, Joanna and her father agreed to meet on Starbase 11 while the Enterprise was docked there. While waiting for her father, Joanna met Admiral James T. Kirk in a bar and the two began flirting with each other, which did not meet the approval of her father. However, soon after Joanna was kidnapped by Jahn as revenge against McCoy for the death of Miri. McCoy set off in search of Joanna and Jahn and discovered that the vaccine developed to prevent the onlies' deaths had mutated and they were dying again. (Untold Voyages comic: "Past Imperfect")
In an alternate timeline in which John Frederick Paxton destroyed Starfleet Command and ended the talks for the Coalition of Planets in 2155, Joanna's mother Jocelyn died in 2246, only one month after giving birth to her. She was raised by her father single-handedly and the two of them had an extremely close relationship.
While she was attending the University of Mississippi in 2264, her father was assigned to the UESS Enterprise as chief medical officer. She was very upset by the fact that they would likely to be separated by dozens of light years for months at a time and regretted that she was at the spaceport to see him off. (TOS - Myriad Universes novel: A Less Perfect Union)