Kevin Thomas Riley and his family lived at the Tarsus IV colony during the famine of 2246. Riley's father was among the 4,000 colonists murdered by order of Governor Kodos. Riley was one of nine surviving witnesses to Kodos' order.

In 2266, Riley was a lieutenant, and served aboard the USS Enterprise. Early in the year, he served as a navigator. While on duty, he was intoxicated by the "polywater" infection brought back from Psi 2000, and became irrational. During this period, he took over engineering and regaled the crew, much against the wills of many hands, with numerous off-key renditions of "I'll Take You Home Again, Kathleen". He also turned off the engines, causing the ship's orbit to spiral into the planet. Sometime after that incident Riley was transferred down to engineering.

Later in the year, the Enterprise ferried the Karidian Company of Players to Benecia. By this point, Riley had been promoted to the communications section. Kirk moved him back to engineering, to protect him from a killer who had already murdered Dr. Thomas Leighton, one of the three remaining eyewitnesses. While he was there, Lenore Karidian attempted to poison him, and might have succeeded had he not been talking to off-duty personnel at the moment the poison took effect, allowing them to realize that Riley was in peril and send help. Recovering in sickbay, Riley discovered that Anton Karidian was really Kodos the Executioner. Riley approached Karidian with murder on his mind, but Kirk was able to talk him out of his vengeance.