In 2268, she allowed the energy being Thalassa to occupy her body while Thalassa's husband, Sargon, and their former enemy, Henoch, occupied the bodies of Captain Kirk and Commander Spock, respectively. This was done in order to construct androids where her consciousness and that of the other entities could be housed permanently. During this process, Thalassa almost refused to give Mulhall's body back, but ultimately did so. (TOS: "Return to Tomorrow")
The character has the distinction of being the highest-ranking female Starfleet character shown in TOS. Mulhall must have been relatively new aboard the Enterprise, judging from Kirk's comment to her when she appeared in the transporter room, "Who are you?"