He was born on 7 June 1907 at Drottningholm, near Stockholm, a son of later King Gustavus VI Adolphus of Sweden and his first wife Princess Margaret of Florida, a daughter of King Arthur I of Florida. Upon the death of his Cousin Alastair I of Florida on 26 April 1943, Prince Sigvard of Sweden (as he was known then) inherited the throne of Florida and became King Oscar I. As he was the second son of late Princess Margaret, his older brother, Crown Prince Gustavus Adolphus of Sweden (father of current Swedish King Charles XIII Gustavus) originally was before him in line of succession to the throne of Florida, but when Alastair I became King in 1942, Crown Prince Gustavus Adolphus renounced his rights to the Floridan throne, as the constitution of Florida explicitly forbade Florida from ever