In 49 BC, as tribune of the people, he strongly supported the cause of Caesar, by whom he was made governor of Hispania Ulterior.
He treated the provincials with great cruelty, and his appointment in 48 BC to take the field against Juba I of Numidia gave him an excuse for fresh oppression.
Cassius punished the leaders with merciless severity, and made the lot of the provincials harder than ever.
Bogud, king of Mauretania, and Marcus Lepidus, proconsul of Hispania Citerior, to whom Cassius had applied for assistance, negotiated an arrangement with Marcellus whereby Cassius was to be allowed to go free with the legions that remained loyal to him.
Cassius sent his troops into winter quarters, and hastened to board a ship at Malaca with his ill-gotten gains, but was wrecked in a storm at the mouth of the Iberus (Ebro), in which he was killed.