Flaccus was appointed to the Agrarian Commission in 129 BC. In 122 BC he became a tribune to assist Gaius Gracchus in implementing an amended version of his policy of citizenship for Italians, making him the only ex-consul to hold the position of tribune.
He went to found a Roman colony, Colonia Junonia, on the ruins of Carthage. When he and Gracchus failed to win re-election in 121 BC, Flaccus led a mass protest on the Aventine Hill, but the consul Lucius Opimius suppressed it brutally, killing Gracchus and Flaccus among many others.
Plutarch describes him as a born agitator. Cicero describes Flaccus as an orator of moderate gifts and comments that his writings reveal him as a student of letters rather than an orator.
He wife is unknown. Flaccus had a son called Marcus Fulvius Flaccus Bambalio. Bambalio married Sempronia Gracchae, the daughter of Roman Tribune Gaius Gracchus. Their marriage resulted in a daughter and only child Fulvia Flacca Bambula, who was Flaccus' only grandchild.