Mignard completed is PhD in comparative criminal law summa cum laude with a dissertation on cybercriminality and repression in 2004, under the supervision of Mireille Delmas-Marty, a professor at the Collège de France.
Mignard has participated in territorial litigation resolution councils with several states, including Chad, Cameroon, and Benin, working under the supervision of French law scholars Alain Pellet and Jean-Pierre Cot.
He also took part in international arbitration cases in environmental law at The Hague and Paris, on behalf of publicly owned companies, or in defense of high-level French athletes such as Christophe Dugarry or Jeannie Longo, as well as reporters, show-business, and artistic personalities.
After her defeat to conservative candidate Nicolas Sarkozy, Mignard went on to become the chair of Ségolène Royal's political action committee, Désirs d'Avenir, a position from which he resigned in September 2011.
In 1993, Mignard ran for to represent Nièvre's 2nd district in the lower house of the French Parliament but was defeated, nonetheless scoring 49.40% in a historically difficult year for socialist candidates around the country, as a conservative wave swept the left-wing parliamentary majority out of power.