Clayton served as an ODST during the Human-Covenant War, serving during many of the major battles of the final days of the war, including the Fall of Reach and the Battle of Earth. After the war, Clayton was among the first recruited SPARTAN-IVs, and passed the selection, training, and augmentation processes with flying colors. Unfortunately, during one of his first deployments as a SPARTAN-IV, Clayton was caught in the blast of an InsurrectionistIED, costing him both of his legs.
Unable to fight but unwilling to allow the UNSC to discharge him, Clayton was transferred to the UNSC Infinity, where he would serve as a mission handler for Fireteam Valiant. However, he was extremely jealous of his able-bodied peers, and gained a reputation for having an irritable demeanor because of this.