[5] He defended Edmond Hervé in the ministers' trial of the Infected blood scandal in 1999, after years of investigations and procedure;[6] Gilles Ménage (former chief of staff of François Mitterrand) in the trial (2005) of the Presidency's abusive wiretappings (1983–1986);[7] and Sébastien de Montessus, former director of mines of Areva, sued by his own ex-boss, Anne Lauvergeon, for alleged spying of her private life (Sébastien de Montessus has been acquitted).
He defended him in front of the Conseil supérieur de la magistrature [fr] (CSM, Higher Judicial Council),[9] and sued two national newspapers as well as Bertrand Tavernier for libel.
[13] He is defence lawyer for Yoo Som-na, the eldest daughter of Yoo Byung-eun[14] and Christine Lagarde, director of the International Monetary Fund[15] Regardless, Maisonneuve is on the plaintiffs' side in some important cases, especially the case of the Fédération nationale de la mutualité française [fr] against Laboratoires Servier (Mediator scandal)[16] and the one of the European Parliament against Marine Le Pen.
[17] On 26 May 2014, Maisonneuve revealed during a press conference that the company Bygmalion [fr], for which he is the lawyer, made bogus invoices as a result of the pressure exerted by the staff of Nicolas Sarkozy and by the UMP: instead of making bills for Nicolas Sarkozy's campaign, Bygmalion was requested to exaggerate or to invent works for the UMP's ordinary activities, because the electoral expenses exceeded the legal spending limit.
Patrick Maisonneuve estimated the total amount of the fake invoices to "more than 10 million euros" and called the demand of the UMP for "financial blackmail".