1796, "science of ideas," originally "philosophy of the mind which derives knowledge from the senses" (as opposed to metaphysics), from French idéologie "study or science of ideas," coined by French philosopher Destutt de Tracy (1754-1836) from idéo- "of ideas," from Greek idea (see idea) + -logie (see -logy). With connective -o- because the elements are Greek and the Greek combining vowel is -o- for nouns of all declensions. Destutt published his Eléments d'idéologie 1801-1815.
The term ideology did not become widely employed in the nineteenth century, however, and I have not found that Emerson ever used it. It was only after the appearance of Karl Marx's long unpublished The German Ideology and Karl Mannheim's Ideology and Utopia in the period between the world wars of the twentieth century that the term became an omnipresent one. [Lewis P. Simpson, "Mind and the American Civil War," 1989]
Meaning "systematic set of ideas, doctrines through which the world is interpreted" was in use in English by 1907, earliest in socialist and communist writing, with reference to class; from 1918 it came to be used of socialism and communism themselves (along with fascism) and later more broadly still.
Ideology ... is usually taken to mean, a prescriptive doctrine that is not supported by rational argument. [D.D. Raphael, "Problems of Political Philosophy," 1970]