Barnhart was a crewmember aboard the USS Enterprise in 2266.
In that year, Barnhart was one of a number of Enterprise crewmembers killed by a creature which had infiltrated the ship and was extracting all of the sodium chloride from its victims. Barnhart's body was found on deck nine, section two. (TOS: "The Man Trap")
Barnhart was played by actor Budd Albright, and was one of many Enterprise crewmembers to perish in the line of duty. For more, see Starfleet casualties (23rd century).
Barnhart died wearing the rarely-seen Starfleet utility vest uniform (which was a reused costume from The Outer Limits), and was carrying his headgear.