Before joining Starfleet, Arex spent ten years in a space merchant fleet. Having never studied at the Academy, he became an officer by field commission after assuming temporary command of a Federation cruiser whose crew was killed during an unexpected Klingon attack; in which occasion Arex managed to outwit the Klingons and disable their vessel. Widely regarded as one of the most skilled navigators of his time; he spent two years as an instructor on the field in Starfleet Academy, and had among his students none other than Pavel Chekov, whose position in the Enterprise bridge would later be assumed by Arex himself following his former student's temporary transference.