By 1632 he was a composer and musician in the royal chapel in Warsaw. In 1645 he became director of music to Charles Ferdinand Vasa, the brother of King Władysław IV. Mielczewski died in Warsaw in September 1651.
His known works are largely in the concertato style, and Szweykowski suggests that the way "in which the words are given full expression" means he is likely to have composed secular vocal works in addition to the surviving sacred corpus. In his massO glorioso domina and one of his instrumental canzonas, Mielczewski quotes popular Polish tunes; the latter is notable for being the earliest documented use of the mazurka in classical music.