Born at Penshurst Place, Kent, he was the eldest son of Sir Henry Sidney and Lady Mary Dudley. His mother was the eldest daughter of John Dudley, 1st Duke of Northumberland, and the sister of Robert Dudley, 1st Earl of Leicester. His younger sister, Mary Sidney, married Henry Herbert, 2nd Earl of Pembroke. Mary Sidney, who upon her marriage became the Countess of Pembroke, was a writer, translator and literary patron. Sidney dedicated his longest work, the Arcadia, to her. After her brother's death, Mary Sidney Herbert reworked the Arcadia, now known as The Countess of Pembroke's Arcadia.