The duo invested the money to ensure the club's immediate return to the Premier League, with the majority of funds being spent on new players for manager Sam Allardyce.
Allardyce brought in 18 new players for the ultimately-successful promotion campaign.
"Both myself and my partner David Gold decided to invest our money into achieving promotion and during the season we invested a further £32.2m of our cash in order to allow the manager to go into the transfer market and buy the players we needed to secure promotion," Sullivan wrote.
"During that period we brought or loaned 18 players, most notably Kevin Nolan, Matthew Taylor and Ricardo Vaz Te, investing a total of £17.1m."
Meanwhile, the Hammers are hopeful of agreeing a deal for the occupancy of the Olympic Stadium in the next couple of weeks.