Verbs

(v)mellowsoften, make mellow“Age and experience mellowed him over the years”
(v)mellow, melt, mellow outbecome more relaxed, easygoing, or genial“With age, he mellowed”
(v)mellowmake or grow (more) mellow“These apples need to mellow a bit more”, “The sun mellowed the fruit”

Adjectives

(s)laid-back, mellowunhurried and relaxed“a mellow conversation”
(s)mellow, mellowedhaving a full and pleasing flavor through proper aging“a mellow port”, “mellowed fruit”
(s)mellow, mellowedsoftened through age or experience“mellow wisdom”, “the peace of mellow age”
(s)high, mellowslightly and pleasantly intoxicated from alcohol or a drug (especially marijuana)

Adverbs

(r)mellowly, mellowin a mellow manner