Sebastian was born heir-apparent to the throne of Portugal, since his birth occurred two weeks after the death of his father. He succeeded to the throne at the age of three, on the death of King John III, his paternal grandfather. Soon after his birth, his mother Joanna of Spain left her infant son to serve as regent of Spain for her father, Emperor Charles V. After his abdication in 1556, she served in the same capacity for her brother Philip II of Spain. Joanna remained in Spain until her death in 1573, never to see her son again.