1742, from ascend + -ance. According to OED, properly "the act of ascending," but used from the start in English as a synonym of ascendancy "state of being in the ascendant, governing or controlling influence."
late 14c., "move upward," from Latin ascendere "to climb up, mount," of planets, constellations, "come over the horizon," figuratively "to rise, reach," from ad "to" (see ad-) + scandere "to climb" (see scan (v.)). Also in 15c. used with a sense "to mount (a female) for copulation." The meaning "slope upward" is from 1832. Related: Ascended; ascending. An Old English word for it was stigan.
word-forming element attached to verbs to form abstract nouns of process or fact (convergence from converge), or of state or quality (absence from absent); ultimately from Latin -antia and -entia, which depended on the vowel in the stem word, from PIE *-nt-, adjectival suffix.
Latin present-participle endings for verbs stems in -a- were distinguished from those in -i- and -e-. Hence Modern English protestant, opponent, obedient from Latin protestare, opponere, obedire.
As Old French evolved from Latin, these were leveled to -ance, but later French borrowings from Latin (some of them subsequently passed to English) used the appropriate Latin form of the ending, as did words borrowed by English directly from Latin (diligence,absence).
English thus inherited a confused mass of words from French (crescent/croissant), and further confused it since c. 1500 by restoring -ence selectively in some forms of these words to conform with Latin. Thus dependant, but independence, etc.