After nearly four years heading the Securities and Exchange Commission, Chairman Mary Schapiro announced on Monday that she will depart the post, effective Dec. 14, 2012.

The White House announced on Monday that President Obama was naming Elisse B. Walter, a commissioner at the SEC, as the new chairman. Walter will not step into an interim post, but will take over the top spot for the foreseeable future. Since she has already been confirmed as a commissioner by the Senate, she will not need to be reconfirmed as chairman, easing the transition.

"I am also pleased to designate Elisse Walter as SEC Chairman after Mary's departure. I'm confident that Elisse's years of experience will serve her well in her new position, and I'm grateful she has agreed to help lead the agency," said President Obama in a statement.

Appointed by President Barack Obama on Jan. 20, 2009, Schapiro is one of the longest-serving SEC chairmen, having held the job longer than 24 of the previous 28 chairmen. Schapiro previously served as a commissioner at the SEC from 1988 to 1994. She was appointed by President Ronald Reagan, reappointed by President George H.W. Bush in 1989, and named Acting Chairman by President Bill Clinton in 1993.

"I want to express my deep gratitude to Mary Schapiro for her steadfast leadership at the Securities and Exchange Commission. When Mary agreed to serve nearly four years ago, she was fully aware of the difficulties facing the SEC and our economy as a whole. But she accepted the challenge, and today, the SEC is stronger and our financial system is safer and better able to serve the American people – thanks in large part to Mary's hard work," President Obama said in a statement on her resignation.

Shapiro left the SEC when President Clinton appointed her as chairman of the Commodity Futures Trading Commission, where she served until 1996. She is the only person to have ever served as chairman of both the SEC and CFTC.

As SEC chairman, Schapiro also serves, for now, on the Financial Stability Oversight Council, the FHFA Oversight Board, the Financial Stability Oversight Board, and the IFRS Foundation Monitoring Board.

“It has been an incredibly rewarding experience to work with so many dedicated SEC staff,”  Schapiro said in a statement. “Over the past four years, we have brought a record number of enforcement actions, engaged in one of the busiest rulemaking periods, and gained greater authority from Congress to better fulfill our mission.”

Among the SEC efforts related to the Dodd-Frank Act during her tenure were implementing a new whistleblower program, strengthening regulation of asset-backed securities, and requiring advisers to hedge funds and other private funds to register and be subject to SEC rules.