Cllr Colin Rayner (Con, Horton and Wraysbury) made the comments about Maidenhead based The Spoore, Merry and Rixman Foundation at a Royal Borough cabinet meeting on Thursday, October 24.
Speaking at the meeting Cllr Rayner said he was concerned the charity was not meeting the needs of the benefactors and was not well-known.
He became a trustee as part of his mayoral duties.
"When I turned up to a meeting they said 'who are you?' I told them the clue was in the chain.
"I have spoken to a number of people, including Theresa May MP, who have never even heard of them.
"We have a duty as councillors and trustees to make sure they meet their requirements.
"There are people in Maidenhead whose children are going to school hungry and these charities can help, they have to open their heart, get out and find the needy."
Deputy leader Cllr Simon Dudley (Con, Maidenhead Riverside) raised his own concerns about the charity's transparency.
He said: "There are no details about the trustees, minutes or accounts on their website, I simply do not think it's acceptable and I would like to review the legal advice they have had."
Cllr David Coppinger (Con, Bray), who serves as a trustee for the charity, defended the organisation against the comments.
He said: "As a trustee, I will not get into a slanging match with the former mayor but I completely disagree with what he has said.
"The charity distributes every penny of the £400,000 they receive every year, every single penny.
"I agree there should be more information on the website but I find the statements which malign an excellent charity extremely unfortunate."
Council leader David Burbage (Con, Bray) said he would write to foundation to take up the points about transparency and legal advice.
The charity said after the meeting it could not comment until after the letter from Cllr Burbage was received.
It was founded in the 17th century. Rents from its properties are used to pay for education and training for children in Bray and Maidenhead.
The cabinet meeting was at Windsor Guildhall.
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