On November 19, 1918, he landed in Dalmatia, which had been occupied by Italian troops following the Armistice of Villa Giusti and in view of the annexation to Italy as agreed in the London Pact.

Here he worked until April 1919 in Muć, Drniš and Šibenik, later moving to Zara by order of Vice Admiral Enrico Millo, Governor of Dalmatia, where Serrentino held the position of head of the passport office.

In the early 1920s he was among the main leaders of the Zara section of the Italian Fasces of Combat and joined the Volunteer Militia for National Security (MVSN), reaching the rank of primo seniore (lieutenant colonel) by 1940.

[4][5][6] After the Armistice of Cassibile in September 1943, Serrentino joined the Italian Social Republic and was appointed Prefect of Zara, a post he held throughout the period of German occupation, from November 1943 until shortly before the arrival of Tito's partisans in October 1944.

He was the highest civilian authority in Zara during the period of German occupation; in the same day he formally took office, the town was hit by the first of the dozens of air raids that would leave it largely destroyed and deserted by the autumn of 1944.