A member of the Socialist Party, he was secretary of state for technical education, deputy for the second and then first constituencies of Essonne and mayor of Évry.

In 1974, at the same time as leaving Paris to settle in Évry, he broke with CERES and joined the Mitterrandist majority of the Socialist Party.

He bid farewell to politics in 2002, leaving his seat as a deputy to the new mayor of Évry, Manuel Valls.

[2] Jacques Guyard was indicted for corruption and influence peddling in the SAGES affair by Judge Renaud Van Ruymbeke in 1992, in connection with contracts awarded between this consultancy close to the Socialist Party and the commune of Évry, of which he was mayor,[3] and for the same charges in the SANE affair by Judge Jean-Marie d'Huy in February 1995.

[5] He was acquitted on appeal in 2001, as the Court of Appeal ruled that, while the remarks in question were indeed "defamatory", Jacques Guyard was acting in "good faith" and "the judge is not bound by the conclusions of a Commission of Inquiry and cannot therefore pronounce on the quality of the investigations carried out by the investigator".