early 15c., "in, of, or pertaining to the mind; characteristic of the intellect," from Late Latin mentalis "of the mind," from Latin mens (genitive mentis) "mind," from PIE root *men- (1) "to think."
In Middle English, also "of the soul, spiritual." From 1520s as "done or performed in the mind." Meaning "crazy, deranged" is by 1927, probably from combinations such as mental patient (1859); mental hospital (1891). Mental health is attested by 1803; mental illness by 1819; mental retardation by 1904.
word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).
Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]
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Harper, D. (n.d.). Etymology of mentality. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/mentality