One of the longest-serving officers in Starfleet history, it is rather ironic that Doctor McCoy never actually intended to join the service in the first place and frequently expressed his dissatisfaction with the entire service! Born in 2227,1
McCoy always wanted to be a GP in a quiet, rural area where he could build a personal relationship with his patients. He joined Starfleet medical school in order to gain medical training, planning to retire after the mandatory five years service. After a brief marriage and bitter divorce McCoy decided to change the direction of his life, remaining in Starfleet for seventeen years after graduation. He became an acknowledged expert in surgery and several fields of space medicine, publishing many papers on the effects of extended service in space on the human body and other subjects.
An outspoken individual, McCoy was never afraid to voice an opinion or tell others when he thought they where wrong - even those in authority over him. With many Captains this would have made for a disastrous situation, but under Captain Kirk, McCoy became an invaluable member of the crew.2 Kirk even joked on occasion that his CMO was the ships self-appointed conscience. Although McCoy did not always get along with all of his shipmates on a day to day basis, he was capable of an incredible level of loyalty and self-sacrifice when the situation called for it.2 When both Kirk and Spock left the Enterprise in 2270, McCoy retired rather than serve under another Captain. He rapidly settled into civilian life, and had no plans to return to Starfleet.
When Kirk returned to take command of the Enterprise in 2271 during the V'Ger emergency, he used the reserve activation clause to forcibly draft McCoy back into the service.3 After the V'Ger situation was resolved Kirk retained command of the ship, with Captain Spock once again as his science and executive officer. McCoy elected to stay with his friends, and continued on board the Enterprise as CMO.
In 2285 the Enterprise was attacked and badly damaged during the Genesis crisis; Captain Spock was mortally injured while conducting repairs on the ship. Before dying, Spock transferred his Katra - a copy of his 'soul' - to McCoy.4 When Spock's regenerated body was recovered from the Genesis planet, McCoy offered to allow the Katra to be moved back into Spock despite the great personal danger this entailed - once again showing the selflessness he was capable of when his friends needed it.5
McCoy retired again in 2293,6 but after some years in civilian life he elected to return in 2300 in order to become a teacher at starfleet medical school. He remained a member of starfleet for over sixty four years after this decision, bringing his total service time to an incredible one hundred and eleven years. By 23647 McCoy was nearly one hundred and forty years old, but he still remained an active member of Starfleet - he even travelled to Farpoint Station on the USS Hood in order to make an inspection tour of the USS Enterprise-D, then making her maiden voyage.1