Swensen was a Starfleet security officer stationed on board the USS Enterprise during the 2260s.
In 2268, he and Bardoli were part of a landing party led by Captain Kirk that was searching for the dikironium cloud creature on Argus X. (TOS: "Obsession")
Later that year Swensen was called to the transporter room where he kept his phaser trained on Jojo Krako when the latter beamed aboard the Enterprise. (TOS: "A Piece of the Action")
Still the same year, he was among the security personnel who fought against Kang's Klingon crew near main engineering, under the influence of the Beta XII-A entity. (TOS: "Day of the Dove")