During World War II, he became the de facto ruler of the annexed Ionian Islands in Greece, and later supported the Italian Social Republic, becoming the mayor of Milan.
On 10 August, the Ionian Islands, except Kythira, were annexed by Italy as part of the Grande Communità del Nuovo Impero Romano (Great Community of the New Roman Empire).
On 16 August, Parini replaced Corfu mayor Spyridon Kollas with lawyer Gerasimos Tryfonas, to sever all administrative ties between the islands and the Greek mainland and the collaborationist government in Athens.
[3] The Ionian Islands did not manage to escape the horrors of the Great Famine of 1941–42, partially due to Parini's refusal to allow the Red Cross to distribute aid in the region.
[4] Following the Fall of the Fascist regime in Italy, Parini departed Corfu in late August 1943 on the yacht Aspasia, while a second ship carried 40 crates of looted art he had collected.
The authenticity of the papers was soon questioned and Parini was deported to Italy, where he was given a 12-year prison term by a military tribunal for war crimes committed during his spell as mayor of Milan; he was released in 1946 in an amnesty.