He attended schools in Mergežerys, Vilnius, Varėna, Griškabūdis, and from 1957 he studied at the Vilnius State Conservatoire, theatre faculty, where he gained a specialisation in acting. However, he worked as an actor for only a couple of years in the then Kapsukas (Marijampolė) Drama Theatre, and later he linked his life with the pen. He worked in the editorial offices of the magazines "Žvaigždutė", "Moksleivis", and the newspaper "Lietuvos pionierius", and in 1973 he became deputy editor of the magazine "Genis". Later, the writer devoted all his time exclusively to creative work.
Martynas Vainilaitis published his first poems in 1953. His debut collection of poems was Vyturiai palydi plūgą (1960). After that, he began to devote his creative energies to children's literature. Already with his first books for the little reader (Pupų pėdas, 1963; Grigo ratai, 1964; Ežio namas, 1967; Varnėnų skudučiai, 1967; Dundulis dunda, 1969), Vainilaitis showed himself to be a capable, creative author, with a good sense of the specifics of his chosen genre and its possibilities. The creative heights of M. Vainilaitis' talent are beautifully marked by the collection Mano volungėlė (My Volungėlė, 1975), for which the poet was awarded the Lithuanian State Prize. All of the poet's books, of which he has written over 30, beautifully bring out the best features of Lithuanian children's poetry: sensitive melody, vividness of image and feeling, attractiveness of characterisations, lyricism, humour, elements of folklore and mythology. The poet was the winner of the 1998 Vilnius Capital Prize and the Vytautas Tamulaitis Prize for his poetic fairy tale "Kalvis", in 2003 his book "Sidabrinė kultuvėlė" (The Silver Cultivator) was named the best children's book of the year, and in 2005 the writer was awarded the Children's Literature Prize. In the same year, M. Vainilaitis was awarded the Officer's Cross of the Vytautas the Great Order.