A member of The Republicans (LR), she succeeded Jean-Claude Gaudin in office, who resigned to focus on his term as Mayor of Marseille.

Vassal previously served as Deputy Mayor of Marseille under Gaudin from 2001 to 2015, when she was elected to the presidency of the Departmental Council of Bouches-du-Rhône, a position she still holds.

In 2001, Vassal was elected to the municipal council of Marseille on the list led by incumbent Mayor Jean-Claude Gaudin,[7][8] of which she became the deputy at city hall.

The first woman to be elected to this function, she defeated incumbent Jean-Noël Guérini;[11][12] it was also the first time in a century that the right-wing had led the department.

In January 2015, following a report from the regional chamber of accounts, the National Financial Prosecutor's Office (PNF) opened an investigation into suspicions of favoritism and illegal taking of interests concerning Martine Vassal and Loïc Fauchon, Chairman of the Société des Eaux de Marseille (SEM, a wholly-owned subsidiary of Veolia).

[18] Two years later, in 2017, she was summoned by the police to be heard on this case on which she had declared in 2014: "It was the Marseille Provence Métropole services which carried out the analysis and we followed their recommendations".

[29] In November 2019, Mediapart published an investigation into the financing of the regional daily La Provence by the Departmental Council of Bouches-du-Rhône after Vassal took the office at the head of the community.

Before the second round of municipal elections in 2020, two running mates of Martine Vassal were filmed by a France 2 journalist, proposing illegal proxies in the 4th and 6th arrondissements of the city, avoiding going to the commissariat.

[35] LR sector mayor Yves Moraine did not deny having proposed such powers of attorney, declaring: “We thought that the conditions for verifying identity would be relaxed given the health situation.