After graduating in law, he became a member of the Special Tribunal for the Defense of the State on 1 August 1931, gaining a reputation as a skilled inquisitory judge.

In October 1943 he joined the Italian Social Republic, becoming once again a member of the Special Tribunal for the Defense of the State when this was re-established on 3 December 1943.

As the minutes of the 25 July session of the Grand Council of Fascism had been lost, Cersosimo had to piece together the events based on newspaper articles and the statements of the defendants, whom he interrogated in prison.

When he tried to interrogate Galeazzo Ciano in the Scalzi prison in Verona, Cersosimo was stopped by the German guards, but was later able to carry out his task after the intercession of Hilde Purwin.

Admirals Gino Pavesi and Priamo Leonardi were also sentenced to death in absentia, for the fall of Pantelleria and Augusta in the summer of 1943.