Being unfamiliar with Starfleet protocols, she had a hard time adjusting to life on Voyager.
She and three other former Maquis, Kenneth Dalby, Chell and Gerron, were selected by Commander Chakotay to participate in Lieutenant Tuvok's Starfleet Academy-style crash-course aimed at bringing former Maquis members unfamiliar with Starfleet up to date. Tuvok had her remove her "festive" headband, as it violated regulations.
During a simulated battle exercise, Dalby ordered her to fire at will and do as much damage as possible to Romulan Warbirds attacking them. As a result, she, along with all the others, "died". (VOY: "Learning Curve")
Background information Edit
Mariah Henley was played by Catherine MacNeal.
Henley's first name was not mentioned in the episode itself, but was in the script.