King has led FanDuel for the past three-and-a-half years, and oversaw the deal that meant FanDuel became part of the Flutter business in May 2018.
He will remain with FanDuel while a search process is undertaken to appoint his successor.
“It has been a privilege to lead FanDuel over the last four years through what has been an incredibly exciting period for the company,” King said. “With FanDuel well positioned for the next chapter of its growth and always an entrepreneur at heart, now is the time for me to take on new opportunities as the next step in my career.
“I have no doubt that the business will continue to go from strength to strength and I wish all of my colleagues the very best for the future as they pursue the next stage of growth.”