In the Perusine War, 41-40 BC, Furnius took part with Lucius Antonius. He defended Sentinum in Umbria against Augustus, and shared the sufferings of the "Perusina Fames" ("Perusine famine"). Furnius was one of three officers commissioned by Lucius Antonius to negotiate the surrender of Perusia, and his reception by Augustus was such as to awaken in the Antonian party suspicions of his fidelity.