Ellis had a distinguished career in Starfleet, having been decorated for bravery under fire. Ellis was on Rigel VII during the Kaylar coup and his actions during that crisis led to Ellis being given the Starfleet Award of Valor. By the late 2260s Ellis had risen to the rank of Lieutenant Commander and served as the first officer of the Constitution class starship USS Zimbabwe in the late 2260s.
During the 2260s, Lieutenant Ralph Garvin - who had been a roommate of Ellis at Starfleet Academy - was killed by the dikironium cloud creature. Because of this, Ellis became deeply resentful of Captain James T. Kirk.
In 2269 the Zimbabwe was attacked by a space vessel which resembled an old German tri-plane. This vessel was piloted by the omnipotent being Trelane. Ellis was called to the bridge, but before he could get there Trelane was able to capture the Zimbabwe. Ellis soon found himself locked in a room with Doctor Leonard McCoy, Commander Spock and James T. Kirk. Ellis allowed his resentment to get the better of him, but Kirk refused to be goaded into fighting with him, and Ellis worked with the Enterprise officers to free the Zimbabwe and the other ships Trelane had captured.
Ellis and Kirk tried to iron out their differences in the ship's gymnasium, however Ellis was called back to his ship before the two men could completely come to terms with each other. Despite this, Kirk felt that Ellis was a good man and good officer, and announced his intention to have Lt. Commander Ellis transferred to the Enterprise. (TOS video game: Judgment Rites)