He is most remembered for turning Ptolemy against his sister and co-ruler Cleopatra, thus starting a civil war, and for having Pompey decapitated and presenting the severed head to Julius Caesar according to some sources.
When Ptolemy and Cleopatra were elevated to the status of senior rulers, Pothinus was maintained as the former's regent.
Rome was also enveloped in civil war, and after his defeat in the Battle of Pharsalus, Pompey sought asylum in Egypt.
Pothinus had neglected to note that Caesar had been granting clemency to his enemies, including Cassius, Cicero, and Brutus.
As it happened, however, Caesar came to emerge as the sole credible contender for his position of power, with Pompey dead and a Roman protectorate installed in Egypt.