Mary L. Schapiro, who has led the Securities and Exchange Commission for four years, said Monday that she plans to step down from her post on Dec. 14, 2012.

President Obama has chosen SEC commissioner Elisse Walter to succeed Schapiro as chairwoman (see Elisse Walter Named as New SEC Chair).

Schapiro became SEC chairman in the wake of the financial crisis in January 2009, succeeding former Congressman Christopher Cox. Amid heavy criticism of the agency’s performance in forestalling the financial crisis, Schapiro aimed to strengthen, reform and revitalize the agency.

She also oversaw a more rigorous enforcement and examination program, and shaped new rules by which Wall Street must play. However, in terms of accounting policy, while she has supported the concept of global accounting standards, she has resisted holding a vote on whether U.S. companies can be allowed to use International Financial Reporting Standards or whether IFRS can be incorporated into the U.S. financial reporting system.

“It has been an incredibly rewarding experience to work with so many dedicated SEC staff who strive every day to protect investors and ensure our markets operate with integrity,” 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.”

Schapiro is one of the longest-serving SEC chairmen, having served longer than 24 of the previous 28. She was appointed by President Barack Obama on Jan. 20, 2009, and unanimously confirmed by the Senate.

During her tenure, Schapiro worked to bolster the SEC’s enforcement and examination programs, among others. As a result of a series of reforms, the agency has become more adept at pursuing tips and complaints provided by outsiders, better able to identify wrongdoers through vastly upgraded market intelligence capabilities, and more strategic, innovative and risk-focused in the way it inspects financial firms.

In each of the past two years, the agency has brought more enforcement actions than ever before,  the SEC noted, including 735 enforcement actions in fiscal year 2011 and 734 actions in FY 2012.

In addition, the SEC engaged in one of the busiest rulemaking periods in decades. Due to new rules now in place, investors can get clear information about the advisers they invest with, vote on the executive compensation packages at companies they invest in, benefit from additional safeguards that protect their assets held by investment advisers, and get access to more meaningful information about company boards and municipal securities.

“I’ve been so amazed by how hard the men and women of the agency work each and every day and by the sacrifices they make to get the job done,” added Schapiro. “So often they stay late or come in on weekends to polish a legal brief, review a corporate filing, write new rules, or reconstruct trading events. And despite the complexity and the intense scrutiny, they always excel at what they do.”

As a result of the Dodd-Frank Wall Street Reform and Consumer Protection Act, the agency has implemented a new whistleblower program, strengthened regulation of asset-backed securities, laid the foundation for an entirely new regulatory regime for the previously-unregulated derivatives market, and required advisers to hedge funds and other private funds to register and be subject to SEC rules.

During Schapiro’s tenure, the agency worked to improve the structure of the market by approving a series of measures that have helped to strengthen equity market structure and reduce the chance of another Flash Crash. Among other things, the SEC for the first time has required the exchanges to create a consolidated audit trail that will enable the agency to reconstruct trading across various trading venues.

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. She 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 on the Financial Stability Oversight Council, the FHFA Oversight Board, the Financial Stability Oversight Board, and the IFRS Foundation Monitoring Board.