mid-15c., also mixte, "consisting of different elements or parts," from Latin mixtus, past participle of miscēre "to mix, mingle, blend" (from PIE root *meik- "to mix," also see mix (v.)). From 1550s as "not comprised in one class or kind, indiscriminate." Of government from 1530s.
Mixed blessing, one with some unpleasant elements, is by 1849. Mixed marriage is from 1690s, originally in a religious context; racial sense was in use by 1942 in U.S., though mixed breed in reference to mulattoes is found by 1775. Mixed motives is by 1736; mixed feelings by 1782. Mixed bag "heterogeneous collection" is by 1895, from the hunting term for an assortment of game birds killed in one outing. Mixed up is from 1884 as "confused," from 1862 as "involved, implicated" (see mix-up). Mixed metaphor, "an expression in which two or more metaphors are confused," is by 1753.
Mixed drink in the modern liquor sense is recorded by 1868; the thing itself is older; Bartlett (1859) lists sixty names "given to the various compounds or mixtures of spirituous liquors and wines served up in fashionable bar rooms in the United States," all from a single advertisement. The list includes Tippe na Pecco, Moral suasion, Vox populi, Jewett's fancy, Ne plus ultra, Shambro, Virginia fancy, Stone wall, Smasher, Slingflip, Pig and whistle, Cocktail, Phlegm-cutter, Switchel flip, Tip and Ty, Ching-ching, Fiscal agent, Slip ticket, Epicure's punch.