Resurrection of the Daleks
Lieutenant Mercer was a crewmember of the Prison Station on which Davros was held as a prisoner.
He was one of the few initial survivors of the Dalek raid on the station. He led the attempt to activate the station’s self-destruct mechanism, but failed. He was later shot and killed by one of Lytton’s mercenaries. (Resurrection of the Daleks)
BEHIND THE SCENES
In Eric Saward’s novelisation of the story, Mercer’s first name is given as Tyler. This was not derived from any information given in the televised version.