How was slavery made legal? Originally, it started somewhere the grew and grew and eventually became a global business - a way of selling souls and making money. Slavery wasn't made legal it just evolved into something else. It began when the English / Americans went to Africa (West Africa mostly)and kidnapped tribe people. They were traded for cowry shells (the currency back then). Slaves were taken back on ships and worked on plantations, the harvested crops been taken on a journey called the Transatlantic Triangle. One man owning another was now common and soon it became a law. Christian beliefs were changed and false doctrines told.