Genaro was born in Corleone, Italy. He came to the United States with his family when he was very young and settled in Bryan. His father was Michael Angelo Genaro, a successful and influential businessman in his own right. John later became a naturalized citizen and was a resident of Dallas for about 50 years, he founded the Texas Produce Company with his father and brother, Lawrence, in 1912. His brother Lawrence died in 1932 at the age of 47 and his father Michael died the following year leaving John to inherit the bulk of his estate which he used to found several prominent investment firms including Genaro Investments Co., and the Lawrence hotel in Dallas. Genaro was also active in numerous civic affairs. He was a thirty-second degree Mason and a member of Scofield Memorial Church.