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That might depend on how you define an omnivore. Are you using a very general definition, like an organism that consumes both autotrophs and heterotrophs? If so, then it may have been some kind of amoeba or paramecium. Or we may go back further, before the rise of eukaryotes, and point to some predatory bacterium.
Depending on how you define a mammal, the earliest ones probably originated somewhere between 200 and 300 million years ago (probably closer to the former). That would have been the time period between which synapsids arose and when their descendants, the crown group mammals, arose. Even today, many animals considered to be carnivores or herbivores can act as omnivores. Bears eat not only meat but berries as well. Deer have been known to eat baby birds. I imagine the same would have been true of their ancestors. So I'd say the earliest mammalian omnivore probably existed around 200 million years ago.