Sammartini's earliest-known symphonies date from around 1732 and the last from 1772; authentic Sammartini symphonies are 67 in number and are generally divided into three phases; early (18 symphonies, 1732-1739), middle (37 symphonies, 1740-1758), and late (12 symphonies, 1759-1772). An additional 75 symphonies attributed to Sammartini are either spurious or lost. One can trace the development of Classical style through this important cycle of works: the early symphonies reveal traces of Baroque influences, whereas the middle-period works dispense with these elements and add two horns to the ensemble. The late symphonies are longer, and yet more wind instruments are added to the texture, and the continuo is retired for good through his separation of the cello and bass parts. In addition to the symphonies, 31 concerti of Sammartini have been authenticated, and Sammartini deserves to be credited with helping foster the form of the classical symphony concertante. His orchestral music has a driving rhythmic profile and certain minor key works look forward to the Sturm und Drang style of the later classical era. Although it is said in some sources that he produced a four-movement symphony in the 1730s, it is clear that he favored three- and even two-movement symphonies throughout his life.
Sammartini also composed a wealth of chamber music, actually comprising the major part of his surviving output -- he only composed three operas, and preferred to contribute an aria here and there to already existing works over composing whole operas. Sammartini's chamber music is of the highest quality and merits revival.