First widely known as a community activist and as the first president of SOS Racisme in the 1980s, Désir subsequently entered politics in the 1990s, first in Génération Écologie then in the PS.

[8] In addition to his committee assignments, Désir was a member of the parliament's delegations for relations with India (2009–2014), the United States (2002–2004) and South Africa (1999–2002).

From 2002 to 2004, he also served on the parliament's delegation to the Joint Parliamentary Assembly of the Agreement between the African, Caribbean and Pacific Group of States and the European Union (ACP-EU).

[9] He was also a member of the Capital Tax, Fiscal Systems and Globalisation Intergroup of the European Parliament, to whom was presented Denis Robert and Ernest Backes's book, Revelation$, in March 2001.

[11] Désir remained head of the party until April 2014, when he stood down after being appointed State Secretary for European Affairs.

In February 2015, he was appointed chairman of the General Affairs Council of the Party of European Socialists, a position he held until leaving office.

[18] Shortly after taking office, Désir and his Italian counterpart, Sandro Gozi, set out a list of priorities for the time after the 2014 European elections, saying the new European Commission should grant maximum flexibility within existing EU budget rules to countries undertaking growth-promoting investments and structural economic reforms.

[19] On the sidelines of an informal U.N. General Assembly meeting on the rising threat of antisemitism in January 2015, Désir joined his German counterpart Michael Roth in appealing for U.N. member states to work together on an international legal framework that would make social network providers share responsibility for the use of their platforms to spread messages promoting violence; the French call for a radical shift in the way governments treat social networking companies such as Facebook and Twitter came two weeks after the Charlie Hebdo shooting in Paris.

[21] Désir was appointed Representative on Freedom of the Media by the Organization for Security and Co-operation in Europe for a term of three years beginning 18 July 2017.

[25] Since 2023, Désir has been part of the Centre for European Policy Studies/Heinrich Böll Foundation High-Level Group on Bolstering EU Democracy, chaired by Kalypso Nicolaïdis.