After nearly four years at the helm, SEC Chairman Mary Schapiroannounced Monday that she will step down on Dec. 14.

Schapiro is one of the longest-serving SEC chairmen, havingserved longer than 24 of the previous 28, according to thecommission. She was appointed by President Barack Obama on Jan. 20, 2009, and unanimouslyconfirmed by the Senate.

“It has been an incredibly rewarding experience to work with somany dedicated SEC staff who strive every day to protect investorsand ensure our markets operate with integrity,” said Schapiro in astatement.

During her tenure, Schapirosaid she worked to bolster the SEC's enforcement and examinationprograms, among others. As a result of a series of reforms, theagency said it is more adept at pursing tips and complaintsprovided by outsiders, better able to identify wrongdoers throughvastly upgraded market intelligence capabilities, and morestrategic, innovative and risk-focused in the way it inspectsfinancial firms.

In each of the past two years, the SEC said it has brought moreenforcement actions than ever before, including 735 enforcementactions in fiscal year 2011 and 734 actions in FY 2012.

In addition, the SEC said it engaged in one of the busiestrulemaking periods in decades. Due to new rules now in place,investors can get clear information about the advisers they investwith, vote on the executive compensation packages at companies theyinvest in, benefit from additional safeguards that protect theirassets held by investment advisers, and get access to moremeaningful information about company boards and municipalsecurities, according to the SEC.

“I've been so amazed by how hard the men and women of the agencywork each and every day and by the sacrifices they make to get thejob done,” Schapiro said. “So often they stay late or come in onweekends to polish a legal brief, review a corporate filing, writenew rules, or reconstruct trading events. And despite thecomplexity and the intense scrutiny, they always excel at what theydo.”

As a result of the Dodd-Frank Wall Street Reform and Consumer Protection Act, theagency said it has also implemented a new whistleblower program, strengthened regulation of asset-backedsecurities, laid the foundation for an entirely new regulatoryregime for the previously-unregulated derivatives market, andrequired advisers to hedge funds and other private funds to register and be subject to SECrules.

During Schapiro's tenure, the agency said it worked to improvethe structure of the market by approving a series of measures thathave helped to strengthen equity market structure and reduce thechance of another Flash Crash. Among other things, the commissionsaid for the first time, has required the exchanges to create aconsolidated audit trail that will enable the agency to reconstructtrading across various trading venues.

Schapiro previously served as a commissioner at the SEC from1988 to 1994. She was appointed by President Ronald Reagan,reappointed by President George H.W. Bush in 1989, and named actingchairman by President Bill Clinton in 1993. She left the SEC whenPresident Clinton appointed her as chairman of the CommodityFutures Trading Commission, where she served until 1996. She is theonly person to have ever served as chairman of both the SEC andCFTC, according to the SEC.

As SEC chairman, Schapiro also serves on the Financial StabilityOversight Council, the FHFA Oversight Board, the FinancialStability Oversight Board, and the IFRS Foundation MonitoringBoard, the SEC said.

There is no word yet on who will likely be appointed to replaceSchapiro.