Star Trek: How EVERY Captain Became Captain

Spock is the longest serving on screen crew member of the original USS Enterprise. He was science officer under Captain Pike and remained in the post under Captain Kirk. He arrived on the Enterprise as an Ensign, rising through the ranks to Lieutenant by the time that he took personal leave to investigate the red bursts that appeared throughout the galaxy.

He was also the Executive Officer for many years, serving as Kirk's second in command. He was on board ship for the entirety of Kirk's five year mission. He would take command of the ship on many occasions, saving the crew more than once.

When the Enterprise returned to space dock for its refit and overhaul, Spock took leave from Starfleet and returned home to Vulcan to complete the Kohlinar Ritual to purge himself of all logic. However, V'Ger called out to him across the cosmos and he returned to the ship again.

For a short time, Spock served on the Enterprise as science officer only, with Kirk in command and Will Decker serving as executive officer. Spock was the one who was able to communicate successfully with V'Ger, allowing the crew to save Earth. Again.

After this maiden voyage of the refit Enterprise, it was returned to space dock once more. At this point, Spock was promoted to Captain of the ship, though the Enterprise, now growing old, was converted to a training vessel. In the hours before the attack on the Enterprise by Khan, aboard the USS Reliant, Spock commanded the Enterprise on a training voyage.