The Master may not have been part of Doctor Who from the beginning, but portents of their arrival were present as early as the First Doctor’s era. Blessed with many great monsters Doctor Who may be, but few of them can hold a candle to the Master. As an evil Time Lord and the Doctor’s personal rival, the Master is arguably – Daleks be damned – Doctor Who‘s defining nemesis. In official terms, the Master, played by Roger Delgado, first appeared in 1971’s “Terror of the Autons,” facing off against Jon Pertwee’s Third Doctor.

https://screenrant.com/doctor-who-the-master-meddling-monk-war-chief-op-ed/