Sergij's father was a naval engineer and Yugoslav diplomat who moved around Europe because of his work, while his mother Ide Jeanrenaud was of an Italian-speaking Triestine of mixed Swiss, Huguenot and Sudeten German descent.
In June 1942, he was arrested by the Italian Fascist authorities of the Province of Ljubljana and interned in the concentration camp in Gonars.
After World War II, he covered several posts in the state administration before becoming the head of the City Historical Archives of Ljubljana.
In 1961 he spent 6 months in Paris studying economic history with Jean Meuvret at the École pratique des hautes études.
For his work, he received several prizes at home and abroad, among them, in 1980, Anton-Gindely Preis from the Institut für den Donauraum und Mitteleuropa in Vienna (Austria).