And the 23-year-old said he was ‘proud to become a custodian of this esteemed’ institution.
Details of his exact shareholding in the club have not yet been disclosed, with Stewart Donald, Charlie Methven and Juan Sartori all retaining a minority stake.
The news came shortly after a Companies House update confirmed that FPP Sunderland’s charge over the club had been satisfied.
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‘I would like to thank Stewart, the Board of Directors and the EFL for their diligence and support throughout the recent process.’ Louis-Dreyfus said.
‘I am proud to become a custodian of this esteemed institution, but I also recognise the significant responsibility that comes with it.
‘Today marks the start of an exciting new chapter in Sunderland AFC’s history and although the current landscape facing football dictates that there are challenges to overcome, 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.’
Frenchman Louis-Dreyfus is the son of former Marseille owner Robert Louis-Dreyfus and becomes the youngest chairman in English football.
Sunderland are currently seventh in League One – a point outside the play-offs and four behind fourth-placed Pompey.
The Black Cats reached the final of the Papa John's Trophy on Wednesday with a penalty shootout victory over Lincoln.