King Midas (from Greek mythology) was a king who wanted to become wealthier than he was. He asked a god to be able to turn to gold whatever he touches. The god Dionysus granted his wish.
But, king Midas obviously didn't think this over. Soon enough he realized that he has no control over his ability and his food and loved ones also turn to gold. (King Midas was lucky that the god Dionysus later absolved him from this curse.)
What would happen to a modern day king Midas who could literally turn anything into gold with his touch - and is able to turn it on and off at will?
Well, first of all he would be a great threat to (inter)national security on many levels. The modern day king Midas would have to avoid getting abducted, killed, enslaved and/or dissected in a laboratory somewhere.
Upon creating mountains gold and increasing the supply substantially, he would discover that gold has become dirt cheap - and his ability nearly worthless. Even in the best scenario for our modern king Midas he would have to enforce a monopoly on the supply of gold and fix the prices.
Even with such a miraculous ability, king Midas would not necessarily become the richest man in the modern world.