Marsilius received his early education in Padua, probably completing his arts degree and, perhaps, even a degree in medicine at the university there.
In 1313 Marsilius became rector of the leading university of his day, the University of Paris. The years at Paris were formative for Marsilius. He must have come into contact with the two most important theologians at Paris during that period, Durand of Saint-Pourcain and Peter Aureol. He certainly met the two leading Averroists, Peter of Abano and John of Jandun.
Marsilius's teaching career culminated with the publication in 1324 of his extensive treatise on political power, the Defensor pacis. Marsilius attacked many of the arguments used to support the political and temporal authority of the papacy. Going beyond this, Marsilius further attacked the absolute authority of the papacy within the administrative structure of the Church. When the work and his authorship became widely known in 1326, Marsilius decided to move outside the area of influence of Pope John XXII, who resided at Avignon in southern France. Marsilius sought protection and patronage from the German monarch Louis IV of Bavaria, who was already in conflict with John XXII.
In 1327 Marsilius took part in Louis's expedition into Italy and was with him at Rome in 1328, when he was proclaimed emperor by the people of Rome. In 1328 Marsilius was chosen vicar of Rome, a position in which he persecuted those members of the Roman clergy who remained faithful to John XXII. When Louis was forced to return to Germany, Marsilius accompanied him. He remained at the imperial court for the rest of his life.
In 1342 he wrote a short work entitled Defensor minor, a restatement of his earlier and better-known work.
Marsilius died in Munich around 1342, still unreconciled to the Church.
The principal idea upon which Marsilius established his political theory was the idea of popular sovereignty. He argued, all power is ultimately vested in the people. The secular monarch exercises his political authority not because he receives it as a divine right but because he derives it from the citizens of the state. The Roman pontiff derives his authority not from God, as Christ's vicar, but from the members of the Church. Political authority in the state is derived from the citizens. Only they, acting as a whole or through a delegated authority, have the right to prescribe laws for the state. In order to ensure peace in the state, it is necessary to have one governing agency, which may be, but does not need to be, a hereditary monarchy. Such a head of state should be elected by the entire community. If the monarch acts against the welfare of the community or its laws, he can be deposed. Stronger limits are placed on the authority of the papacy, a subject treated in the second book of Defensor pacis. According to Marsilius, the papacy has no authority in temporal affairs. Even in the Church, authority was to be shared with the bishops. Ultimately pope and bishops were to be answerable to the members of the Church.
There are no records on Marsilius married life and it is probably that he died as a bachelor in the court of Louis IV.