Kyril Louis-Dreyfus has become Sunderland chairman at the age of 23 after his takeover was approved by the English Football League.
The Swiss is the son of former Marseille owner Robert Louis-Dreyfus and becomes the youngest chairman in English football.
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.
It ends a protracted sale by former Eastleigh owner Donald, who became the club's majority shareholder when he bought the Black Cats from Ellis Short in May 2018.
Donald had been trying to sell the Black Cats for more than a year and stepped down as chairman in July.
Louis-Dreyfus acknowledged that there were "challenges to overcome" due to the "current landscape facing football" but said there was cause for optimism.
"I am proud to become a custodian of this esteemed institution, but I also recognise the significant responsibility that comes with it," he said.
"I am confident that together we can weather the present storm and put solid foundations in place to bring sustainable and long-term success to the club."
Charlie Methven and Juan Sartori will also retain a minority shareholding alongside Donald who said the club was now debt free.
Sunderland are seventh in League One, a point outside the play-offs, and reached the final of the Papa John's Trophy on Wednesday with a penalty shootout victory over Lincoln.