The news follows the death of non-executive director and former chairman Richard Hambro on Saturday April 25 after battling cancer.
Simon, who joined the board in November 2007, is the fourth senior management figure to leave Money Portal this year.
Group chief financial officer Nigel Ward resigned in March, six months after joining the firm, while co-founder and executive vice chairman Richard Craven and managing director Chris Edge announced they were leaving the group in February.
Money Portal recently announced that it is considering a capital restructure after being approached by potentially new and existing shareholders.
The firm has appointed Fenchurch Advisory Partners to review and evaluate investment proposals. It confirmed it would be open to an outright sale.
Money Portal chief executive Mark Lund says there are no immediate plans to appoint a replacement.
He says: “Peter has stepped down because he has huge other commitments. He is still a shareholder and has appointed someone to come along to observe the board meetings and report back to him. There is no planned replacement at the moment. I think we will get through our corporate activity and concentrate on that then.”