At about the age of 18, on April 26, 1615, Marini was appointed violinist at St. Mark's in Venice, where his boss would have been Claudio Monteverdi. He returned to Brescia by 1620 to become music director of the Accademia degli Erranti there, but on January 30, 1621, he is listed as a player in the court orchestra of the Farnese family in Parma.
This pattern of staying only a short time at jobs held all through his life until he was quite old. Between 1623 and 1649, he worked at the Wittelsbach Court at Neuberg an die Donau as concert master, but during that time he also shows up in Brussels (1626 or earlier), Düsseldorf (mid 1620s, 1640, and 1644 - 1645), Brescia, Padua (1634), and Venice. Either he held the Wittelsbach job sporadically, or the Wittelsbachs were especially lenient in granting their star violinist leave to travel. In 1649, he received an appointment as maestro di cappella in Santa Maria della Scala church in Milan. His next appointment was as director of the Accademia della Morte in Ferrara (1652 - 1653), and he was living in Venice in 1654. He had a position in Vicenza during 1655 and 1656, and after that seems to have alternated between Brescia and Venice.
Although he wrote vocal music, which tends to be rather stilted, his instrumental music was justly famous and represents the most advanced writing of its type for the time, insofar as playing technique is concerned. He clearly wrote string tremolos at least eight years before Monteverdi is known to have "invented" them. While his early works entailed sinfonias and sonata, there is no clear definition between the two forms, but his later music there has clearly developed a difference: The sinfonias are the lengthier, more varied works. He was one of the first to develop a clear type of "solo" writing for instruments and to promote the format of solo and accompaniment. Another of his innovations is scordatura, or the unusual tunings of the strings of an instrument to allow for unusual chord effects and different resonances of the instrument. Moreover, his writing is skillful, shows an uncommonly fine melodic gift, and has a rhythmic flexibility that relieves the rhythmic monotony of other early Baroque instrumental music. His harmonies, in the later works, become bold with unusual use of chromaticism and dissonance. His music, much of which was in print, was widely circulated and very influential on his contemporary violinists and composers.