He was a member of the Party of Rights and supporter of Yugoslavist ideas advocated by bishop Josip Juraj Strossmayer.
In 1895, Trinajstić established a branch of the Istarska posujilnica u Puli [hr] building society in Pazin and ran it.
In 1915, after beginning of the World War I, Trinajstić left the country and joined the Yugoslav Committee led by Ante Trumbić.
After establishment of the Kingdom of Serbs, Croats and Slovenes, he was appointed a member of the country's provisional legislative body, the Temporary National Representation as a representative of Istria.
Trinajstić left politics after he was disappointed by provisions of the 1920 Treaty of Rapallo which attempted to resolve the Adriatic Question by awarding Istria to the Kingdom of Italy.