He worked one year at Henri Emmanuelli staff at the Assemblée nationale and resumed his teaching between 1993 and 1997.

He was Senior Research Fellow at the Centre national de la recherche scientifique between 2002 and 2004, working on ante-marxist socialism.

In a 2000 report co-authored with Arnaud Montebourg, Peillon alleged that Monaco had lax policies with respect to money laundering, including within its famed casino, and that the government of Monaco had been placing political pressure on the judiciary, so that alleged crimes were not properly investigated.

In a clear challenge to the party's free-market wing, the three called on Socialists to "fight more effectively and resolutely against the ferocity of the new capitalism and the excesses of deregulation.

"[1] In 2007, Peillon joined Ségolène Royal's campaign team as a spokesperson, alongside Najat Vallaud-Belkacem and Montebourg.