Baroness Shephard of Northwold, who was Conservative MP for South West Norfolk 1987-2005, was one of four members of the commission who resigned over the weekend.
The former chairman Alan Milburn said the whole board was standing down because the government was not doing enough to tackle poor social mobility.
In Parliament, the leader of the Liberal Democrats Sir Vince Cable said the resignation of a "highly respected" person showed how serious the concerns of the board must have been.
Labour's education spokesman Angela Rayner said: "When a former Tory education minister resigns from a commission set up by a Tory government this goes beyond politics."
The education minister Robert Goodwill said the government is still committed to improving social mobility.
Baroness Shephard hasn't said why she decided to resign.
In a statement Baroness Shephard said she was "sad to leave the commission but believe that this... is the right time for me to do so", but she did not elaborate on the reasons for her decision.