Posidonius was born around 135 BCE in Apamea in Syria, studied under Panaetius in Athens, and eventually settled in Rhodes, where he ran a school and received visitors including Cicero (who studied with him) and Pompey (who visited him on military campaign and reportedly stood at attention while Posidonius lectured from his sickbed).
He was, by ancient consensus, one of the most learned men of his time. His range covered geography (his measurement of the earth's circumference was influential, though its accuracy depends on which stadion is assumed), astronomy (he calculated the size and distance of the sun, less accurately), history (his now-lost Histories covered the period from 146 BCE to his own time), ethnography (his accounts of the Celts are an important source for what we know of pre-Roman Gaul), and the entire Stoic philosophical curriculum. He was, in the ancient phrase, polymathēs — having learned many things.
His philosophical contributions are harder to pin down because his work survives only in fragments. Galen reports that Posidonius dissented from Chrysippus on the psychology of the passions — Chrysippus had treated passions as mistaken judgments, full stop; Posidonius, drawing on Plato, argued that the soul has irrational parts that contribute their own motive force. This puts him outside the strict early-Stoic line on the unity of the soul.
He also seems to have developed an extensive cosmology grounded in the doctrine of sumpatheia — cosmic sympathy, the idea that the entire universe is a single living organism whose parts are causally connected. The doctrine has both physical implications (it grounded ancient theories of how the moon affects tides, for example) and ethical ones (it grounds the Stoic claim that individuals are organically related to the whole, and so cosmopolitan duty is rooted in metaphysics, not sentiment).
He died around 51 BCE. The Stoa did not produce another systematic thinker on his scale; the next phase of the school is the Roman Stoa — Musonius, Epictetus, Seneca, Marcus — which is closer to practical ethics than systematic philosophy.