mellow has 6 distinct synonyms spread across 6 of its 8 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
slightly and pleasantly intoxicated from alcohol or a drug (especially marijuana)
Synonyms, antonyms and the broader and narrower relations for mellow come from Princeton University’s WordNet. Full sources and licences.
soften, make mellow
“Age and experience mellowed him over the years”
become more relaxed, easygoing, or genial
“With age, he mellowed”
make or grow (more) mellow
“These apples need to mellow a bit more”
in a mellow manner