Together with Engelbert Besednjak, Lavo Čermelj and Ivan Marija Čok, he was the most influential representative of the Slovene émigrés from the Slovenian Littoral during the 1930s.
He was born as Josip Wilfan in a Slovene-speaking upper middle class in Trieste, which was then the largest port of the Austro-Hungarian Empire (now in Italy).
When he was a teenager, his father moved to the Dalmatian coastal town of Dubrovnik, where Josip finished a Croatian language high school.
In Vilfan's view, such coexistence could only be assured on strong local autonomy, a liberal democratic reform of the State, and clearly defined and enforced linguistic rights.
Between 1909 and 1917, he was member of the Trieste City Council, representing the United Slavic National List, which was under the undisputed hegemony of the Slovene Liberals.
At the end of World War I, when the Austro-Hungarian Monarchy collapsed, Wilfan was in Ljubljana, where he was among the founders of the State of Slovenes, Croats and Serbs.
However, after the Treaty of Rapallo assigned the entire former Austrian Littoral to the Kingdom of Italy, Vilfan argued for a policy of agreement with the new State authorities.
Together with Engelbert Besednjak and Virgil Šček, Vilfan emerged as the foremost leader of the Slovene and Croatian national community in the Julian March.
He personally visited the Italian Fascist dictator Benito Mussolini on three occasions (in 1922, 1924, and 1928), in order to persuade him to undertake a more conciliatory policy towards national minorities.
By the mid-1920s, Vilfan's policy of collaboration with the authorities and of passive resistance to Fascist Italianization became increasingly challenged by the ranks of his own National Liberal fraction.
During this time, he closely collaborated with the Christian Socialist politician Engelbert Besednjak for the internationalization of the question of South Slavic minorities in Italy.