Born at Sampierdarena, near Genoa, Valletta was a lecturer in economics before he joined Fiat in April 1921: as a result of his academic qualifications and background he was often known to colleagues and in the trade as "Il Professore". He became director in 1928 and CEO in 1939.
In the upheavals that followed the collapse of the Mussolini regime, Valletta found himself expelled from the company by the powerful unions which considered that he had been sympathetic to the Fascist regime. However, in 1946 he was recalled and nominated as Company President. He presided during two decades of rapid expansion as small Fiats proliferated on Italian streets, and he lived out the injunction of the company's founder to "make Fiat greater, giving more working opportunities to the people, and producing better and cheaper cars". Valletta continued as Chairman of Fiat until, at the age of 83, he retired in April 1966 to be succeeded in that post by the founder's grandson, Dr. Giovanni Agnelli.
He was appointed senator for life in December 1966. The Italian president's citation described Valletta as "the first Fiat worker, and one of the great men who most contributed to the Italian economic miracle and to the welfare of the country".