Cook practiced corporate and securities law as a partner of Winston and Strawn, a prestigious Chicago law firm. He was also active in the Republican Party. So was his father who was chief fund-raiser in Nebraska for Nixon's presidential campaigns in 1968 and 1972.
In 1971 Richard Nixon appointed Cook as general counsel of the Securities and Exchange Commission (SEC). On 3rd March, 1972, Cook replaced William J. Casey as chairman of the SEC. He was the youngest man ever to head a federal regulatory agency.
Cook resigned only ten weeks after he took office when it was disclosed that he had altered an SEC complaint against financier Robert Vesco after discussions with Maurice Stans, the President's chief campaign fund-raiser.