Rockefeller amassed his fortune from the Standard Oil company, of which he was a founder, chairman and major shareholder. By the time of his death in 1937, estimates place his net worth in the range of US$392 billion to US$663.4 billion in adjusted dollars for the late 2000s (decade), and it is estimated that his personal fortune was equal to 1.53% of the total U.S. annual GDP in his day. When considering the real value of his wealth, Rockefeller is widely held to be the wealthiest person in history.
There were also a couple of Romans that were obscenely rich too. You are right though, I had not heard of your Mali king and he was clearly pretty rich. Thanks