Will Mason was a Communications Officer in the Combat Information Center (CIC) aboard the USS Nathan James. His rank was Ensign (O-1), which means he was most likely a recent graduate of the United States Naval Academy.
Mason was shown to be hesitant in his duties, specially when questioned about what he saw or heard on the CIC equipment. However, he stands by his impressions and admits when he is unsure. He is one of the crew members assigned to listen to distress calls from around the world in an attempt to get news about any of the crew's loved ones.
He was also a key member in detecting things on radar, most noticeably with tracking the Immunes' submarine.
In the premiere episode of Season 3, he was captured in Vietnam by pirates along with 6 other members of the Nathan James. During the ambush at a night club, he was shot in the torso. In S3E2, "Rising Sun", he is shown to be bleeding profusely. Doc Rios begs their captors to give him some medical equipment so he can treat him, but the captors grab Mason instead and drag him off to an unknown location. At the end of the episode, it shows Mason on a table being drained of his blood (for unknown reasons at the time). It can be presumed based off of the amount of blood he had already lost (from the bullet wound) and his declining condition before they drained his blood, that they killed him.