A member of a prominent Leftist family,[1] she first became an activist as a student with the Fédération indépendante et démocratique lycéenne.

[3] She is currently Vice-President of the Regional Council of Île-de-France, administering the housing portfolio, since 26 March 2010.

On 30 November 2013, she is elected National Secretary (in French secrétaire nationale) of Europe Écologie – Les Verts (EELV).

[4] On 11 February 2016, Cosse was appointed to French President François Hollande's cabinet as Minister for Housing and Sustainable Habitat.

She resigned from her position as National Secretary of Europe Ecology – Les Verts on the same day.