Doctor Mercer was a Human physician in the 22nd century.

In the 2150s, Mercer became affiliated with the xenophobic terrorist group Terra Prime. Working with John Frederick Paxton, Terra Prime's leader, Mercer was responsible for the creation of a binary clone from the genetic material of Commanders Charles Tucker and T'Pol on the Orpheus Mining colony on the moon. Paxton intended to use the baby to incite xenophobic reactions among Humans and then destroy her. Among his medical technicians were Susan Khouri to monitor the baby.

After Khouri had a change of heart and defected from the group, Paxton questioned Mercer's loyalty, and began to suspect that Mercer was becoming too attached to the baby as Khouri had. Believing that Mercer may be a liability, Paxton had Daniel Greaves and his men kill Mercer, making his death made to look like a mining accident. (ENT: "Demons")

Mercer was played by Patrick Fischler.
In the final draft script of "Demons", Mercer was described as being in his "40's" as of 2155.