Pierre Mussard studied law at the Academy of Geneva and at the Universities of Halle and Valencia. He passed the official law exam in Geneva in 1717. At the instigation of his father Bénigne Mussard, he was appointed honorary professor of law at the Academy of Geneva in 1719. He resigned in 1722 in favour of a political career. The government then decided to create two law chairs for guaranteeing that natural law would henceforth be taught at the Academy. The two newly appointed professors were Jean-Jacques Burlamaqui and Jean Cramer. Mussard was elected to the Council of Two Hundred (1721), the Council of Sixty (1729) and the Small Council (1735). He served three times as syndic (1750-58) and once as first syndic (1762). During the political unrests in 1734-38, he participated in the committee appointed by the Genevan government to respond to the bourgeoisie's demand to restore the former prerogatives of the General Council of the republic. Together with Burlamaqui and Jacob de Chapeaurouge, he exercised a preponderant influence over the final report of the committee, which referred to the social contract theory in order to reject the demand.

No scholarly publications are known. The inaugural lecture on the use and excellence of natural law Mussard presented as honorary professor at the Academy was published in 1720.