In its most dramatic market intervention in years, the U.S. government seized two of the nation's largest financial companies, taking direct responsibility for firms that provide funding for around three-quarters of new home mortgages.

Treasury Secretary Henry Paulson announced plans Sunday to take control of troubled mortgage giants Fannie Mae and Freddie Mac and replace the companies' chief executives. The Treasury will acquire $1 billion of preferred shares in each company without providing immediate cash, and has pledged...