This is a contentious issue.
The first story where a Time Lord called "the Master" appears is "Terror of the Autons" - Season 8 (Third Doctor). That much is agreed on. Some sources and spin-off media back this up.
However, other sources and spin-off media say that the Monk ("The Time Meddler"/"The Daleks' Master Plan") and/or the War Chief ("The War Games") are/is the same Time Lord as the Master, just not called "the Master".
The character "the Master" in ""The Mind Robber" (2nd Doctor) is unrelated to any of these & was not a Time Lord. This character is sometimes described by the longer phrase "the Master of the Land of Fiction", to make the distinction clearer.