Porter was born in Norwich, a younger son of Edmund Porter, prebendary of Norwich and chaplain to Thomas Coventry, 1st Baron Coventry. His mother was Mary, daughter of Sir Charles Chiborne of Messing Hall, Essex and his first wife Jane Spilman. Much of what we know of his early life comes from his own colourful later account, given to his friend Roger North, which North believed to be largely true. During the Second English Civil War, while he was an apprentice, he took part on the Royalist side in the serious rioting in Norwich in 1648. He was pursued by a troop of Parliamentary soldiers, and escaped by seizing a child and pretending to carry it to safety. He fled to Yarmouth and took ship for Amsterdam; there he first trained as a soldier, then ran a tavern. After about five years he judged it safe to return home.