Giannini first started out with founding Bank of Italy in 1904. It was a bank for hardworking immigrants that other banks did not care for. The first day the bank opened deposits totaled $8,780 and within a year this has ballooned to more than $700,000. By 1916, Bank of Italy expanded to open several other branches. Giannini believed branch banking was essential to stabilizing banks during tough times so he went on and bought banks across California. Eventually, there were over 500 branches across the state under the Bank of Italy name.
In 1928, Giannini approached the president and chairman of Bank of America Orra E. Monnette regarding a merger between the two financial institutions. Giannini and Monnette agreed on sticking with Bank of America since the name best represented the broader mission that the new bank would follow. The result of that merger is the Bank of America that exists today.
Giannini is also credited for inventing modern banking practices followed today, pioneering the holding company structure, and establishing one of the first of modern trans-national institutions.