He was subsequently accused of "having participated in the creation and organisation of the Groupe ouvrier et paysan",[7] and on the night of 7 October 1939, he was arrested.

On 20 February 1940, he was ejected from the Legislature, and on 3 April 1940, he was sentenced to five years in prison, the loss of his civil and politic rights, and a fine of 4000 francs.

Freed on 5 February 1943, three months after Operation Torch, he was summoned by the CGT to be a member of the consultative commission of the interim government of Alger.

He participated in the foundation of the Social security system: health insurance, pension system, child benefit, and broad reform of French labour laws, through the creation of employee representative committees, occupational medicine, regulation of overtime, and a change of statute for miners.

The Communist Party organized his funeral, with visitation at the Metalworkers House (now called Maison des métallos) at the CGT siege; historian Michel Étiévent later estimated that a million people attended.

[10] Nicknamed the "ministre des travailleurs" (Minister of the workers), Ambroise Croizat is permanently associated with the major laws concerning Social security in France,[11] notably the creation and administrative organisation of pension funds, and the special officials' treatment.