The former owners of Birmingham City and lifelong West Ham fans will have full commercial and operational control of the club and they have also pledged that they will back, rather than sack, Gianfranco Zola.
An official statement on the official site from Sullivan reads:
“I enjoyed running Birmingham City for 16-and-half years, but everyone there knew my true love was always West Ham United.
"It will be an immense privilege to lead this great football club and more importantly its supporters.
"Our first priority has to be securing the Premier League status of West Ham. I believe the players at this club have shown great commitment in trying circumstances and the new board and I will get behind them in every way we can.
"We will also look to bring in new players to supplement the squad where needed once we have met with the manager."
"I would like to thank former executive chairman Andrew Bernhardt and CB Holding who assumed control of the club in difficult circumstances through no fault of their own.
"Our long-term aim will be to put the club on a stronger financial footing. I believe with our new board we have the expertise and experience to do just that and bring the good times back to this great football club.
"West Ham United need stability after all the recent upheavals. We appointed four managers and parted company with two at Birmingham in 16 years. We believe in our managers and give them the time and support they need.
"The club is now back in the hands of East Enders, people who understand the community and its passion for the Hammers. I believe that depth of feeling will also bring us through what has been a difficult period.”