Corporal Palmer was a UNIT soldier who served under Brigadier Lethbridge-Stewart. He was seconded to UNIT during the Master's attempt to take control of Kronos using TOMTIT (TV: The Time Monster, PROSE: The Shadow in the Glass).
By 2001, he had been promoted to Colonel and was a prominent figure in UNIT's UK branch. He asked for the Brigadier's insight into strange imps surrounding the village of Turelhampton, witnessed by Claire Aldwych, resulting in the Brigadier, Claire and the Sixth Doctor exposing a branch of the Fourth Reich led by Adolf Hitler's son. (PROSE: The Shadow in the Glass)