Kyril Louis-Dreyfus has become Sunderland chairman at the age of 23 after his takeover was approved by the English Football League.
Former Sunderland owner Stewart Donald will keep a minority shareholding.
"Kyril's commitment, acumen and integrity convinced us to accept his proposal," Donald said.
"His vision and desire to bring success back to Sunderland was obvious from the outset and his bid is the one that we feel gives the club the best chance of long-term success and sustainability."
An agreement was reached for Louis-Dreyfus to buy a controlling interest in Sunderland in December, but the League One club had to wait for the EFL to give the deal the green light.