Sullivan first came to public attention following his controversial use of Twitter to discuss West Ham United's club affairs and transfers.
[5] Former West Ham manager, Sam Allardyce, blamed owner David Sullivan for giving the information to his son and for allowing him to tweet it.
This discussed transfer targets and fees and was pulled from the club's official website on the orders of West Ham's managing director, Karren Brady after she had been inundated with complaints from supporters.
[9] In Sullivan's first year, he led an application to advance the club from the part-time third division to the Women's Super League; it was West Ham's first appearance in the top competition.
[13] In November 2021, following Daniel Křetínský's purchase of 27% of shares at the club, Sullivan was appointed as a director at West Ham United.