mid-15c., from mix (n.). Mixed blessing from 1933. 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 bag "heterogeneous collection" is from 1936. Mixed up is from 1884 as "confused," from 1862 as "involved."
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.