Jacini graduated in law in 1908 at the University of Genoa, but in the following years he concentrated on historical studies and political activity. He collaborated with the catholic magazine Il Rinnovamento and in 1911 he was elected in the Municipal Council of Milan. He wasn't confimed in 1914 but he was instead elected in the Provincial Council of Milan.
He fought in the Weltkrieg as an officer and after the defeat of Caporetto in 1917 he was appointed as liaison officer with the british expedition corp.
After the end of the war in 1922 he was once again elected in the Municipal Council of Milan; from his position, he advocated an increased Papal control over the Italian Federation. Criticized for his opinions by those who wanted a stronger autonomy for the separated entities of the federation, he left Milan for Rome in 1928, when he wasn't confirmed in the Municipal Council of Milan. In Rome his influence grew stronger and he gained the trust of Pope Pius XI. When the Vatican attained a better control over the governement, the Pope decided to appoint his loyal friend Stefano Jacini as Prime Minister on 1935.
Stefano Jacini married Elisabetta Borromeo Arese (born in Milan, Italian Federation, June, 4 1888), daughter of Count Don Gilberto and Rosanna Leonardi, on January, 7 1909 in Milan. They have one son, Giovanni, born in Milan on Januray, 4 1910.
He is the nephew of another Stefano Jacini, a famous politician, economist and minister for the Public Works of the Kingdom of Italy. Thanks to his diplomatic skills and his knowledge of german, he was one of main responsible for the italo-german alliance against Austria in the war of 1866.
- Un conservatore rurale nella nuova Italia (A rural conservative in the new Italy), 1926
- Il tramonto del potere temporale nelle relazioni degli ambasciatori austriaci a Roma (The decline of the temporal power of Austrian ambassadors in Rome),1931