Born in Worcester, Massachusetts, the son of Pierre-Basile Mignault and Catherine O'Callaghan, he received a Bachelor of Civil Law degree from McGill University in 1878. He was called to the Quebec Bar in 1878. He then proceeded to practice law in Montreal.
His “Droit civil canadien” (1885–1916), a nine volume work on Québec’s civil law, and its relation to the French tradition, continues to be cited by the courts, including the Supreme Court of Canada.
As a judge on the Supreme Court of Canada (1918–1929), Mignault also had a key role in countering a long-standing centralising tendency in Canadian private law, and in increasing the Court’s sensitivity to the subtleties of Québec’s legal tradition. Mignault saw Québec private law as “surtout fille de la France coutumière” (particularly the heir of French pre-revolutionary customary law), yet also a meeting place for a diversity of philosophical and cultural approaches (“Le Code Civil au Canada” in Le Code Civil 1804-1904, Livre du Centenaire, v.2, Paris: Arthur Rousseau. 725).
He is buried in Notre Dame des Neiges Cemetery. Parc Mignault in Montreal is named in his honour. Armand Marin's biography "L’honorable Pierre-Basile Mignault" (Montréal: Fides) was published in 1946.