'mortalist' originates from English, ultimately derived from Latin; specifically from the Latin word 'mortalis' (via Old French 'mortal' and Middle English 'mortal'), and the English suffix '-ist' (from Late Latin/Greek formations) where 'mort-' meant 'death' and '-ist' denotes 'one who believes in or practices'.

'mortalist' changed from the Latin adjective 'mortalis' into Old French 'mortal' and then Middle English 'mortal'; the modern English formation added the suffix '-ist' to create 'mortalist', giving the sense 'one associated with or holding the view about mortality'.

Initially, 'mortal' meant 'subject to death'; over time the derived noun 'mortalist' came to mean specifically 'a person who denies the immortality of the soul' (and by extension, more rarely, 'one who studies mortality').