As a politician, he has been a member of two left-wing parties, first Proletarian Democracy and then the Communist Refoundation Party; in Milan's mayoral election, he was endorsed by a large left-wing coalition, after winning the primary election of the centre-left coalition with the strong support of Nichi Vendola's Left Ecology Freedom.

After being a volunteer in the Red Cross and a student in Medicine, a worker in a chemical factory, a teacher in the Beccaria youth detention centre, and an employee in a bank, he finally turned to political and law studies, getting a double degree in Political Sciences and Law,[3] and joining his father's firm.

[4] He also participated in international trials, most notably in defence of Abdullah Öcalan, leader of the Kurdish militant organization PKK.

[3] In 1995 Gian Domenico Pisapia died, and Giuliano refused to candidate for regional administrative elections to focus on his father's law firm.

A number of intellectuals and notable people from the cultural and political elite of Milan immediately expressed their support of Pisapia.

On 14 November he ran for the primary elections of the center-leftist coalition led by the Democratic Party, with the support of Nichi Vendola's Left Ecology Freedom, and unexpectedly won (receiving 45% preferences) despite not being an actual member of the PD.