The ratings agency Standard & Poor’s said late on Monday that its president, Deven Sharma, who has become the public face of the firm in the wake of its historic downgrade on the United States’ long-term debt rating, will step down and leave the company by the end of the year.

The decision by Mr. Sharma to resign comes as the ratings agency is under pressure from several fronts, including an inquiry by the Justice Department into its ratings of subprime mortgage securities and a push by activist investors to break up its parent company, McGraw-Hill.

Mr. Sharma will be replaced by Douglas Peterson, a top executive at Citigroup, the company said on Monday.

The management change had been in the works for months and was unrelated to either the Justice Department’s inquiry or to the emergence of the activist investors, Jana Partners and the Ontario Teachers Pension Plan, according to people briefed on the matter.

Mr. Sharma, 55, will step down on Sept. 12, but will remain with the company through the end of the year to help oversee McGraw-Hill’s review of its businesses.

His replacement, Mr. Peterson, 53, is currently the chief operating officer of Citibank, the banking unit of Citigroup.

“We are pleased to welcome Doug to the important role of president of Standard & Poor’s as it continues to build on the enhancements of recent years and accelerates global growth,” Harold McGraw III, McGraw-Hill’s chief executive, said in a statement.

He added, “As we welcome Doug, I particularly want to thank Deven for his dedicated leadership of S.&P.”

One of the most recognized names in finance, Standard & Poor’s is composed of two separate businesses. One is its credit rating agency, a vital cog in global capitalism that monitors the corporate world’s debt issuances. The other is a unit that manages its index products like the Standard & Poor’s 500-stock index.

The ratings agency’s decision to downgrade the United States’ long-term credit rating to AA+ from AAA on Aug. 5 set off a storm of controversy, including criticism by President Obama and Treasury Secretary Timothy F. Geithner. The decision contributed heavily to the worst drop in American stocks since the financial crisis three years ago, as well as volatility that continues to whipsaw the markets weeks later. The other big ratings agencies, Moody’s and Fitch, maintained their top-tier rating on United States debt.

At the same time, the agency is being investigated over whether it improperly rated mortgage securities in the years leading up to the financial crisis. Standard & Poor’s, along with the other major ratings agencies, gave their highest ratings to bundles of troubled loans that appeared less risky during the housing boom, but have since collapsed in value.

Since the financial crisis, the agencies’ business practices and models have been scrutinized by Congress, and Standard & Poor’s is also being investigated by the Justice Department, people briefed on the matter have previously said. At issue is whether the agency’s independent analysis was driven by profits. The Justice Department inquiry, which began before the Standard & Poor’s downgrade of the United States’ debt, is centered on whether analysts’ decisions to assign securities a low credit rating on subprime mortgage loans were overruled by business managers.

Meanwhile, the activist investors pushing for change at McGraw-Hill have recommended that Standard & Poor’s ratings business appoint a “well-known independent oversight figure” to handle government relations.

But people briefed on the matter said Mr. Sharma had been considering stepping down well before the latest attacks on the company. They say that Mr. Sharma first began pondering his options after McGraw-Hill announced last November that Standard & Poor’s would be split into its two component businesses.

That corporate reorganization left Mr. Sharma with less responsibility, leading him to ask about his odds of eventually becoming the company’s chief executive. He was informed that such a move would be unlikely, prompting his desire to leave, one of these people said. Then McGraw-Hill began a search for his successor at Standard & Poor’s, this person added.

Mr. Sharma has been with the company for just under five years, and became president of Standard & Poor’s almost exactly four years ago. His appointment coincided with the beginning of one of the most controversial and tumultuous periods in the history of the 151-year-old company, which was acquired by McGraw-Hill in 1966.

McGraw-Hill’s board eventually signed off on his resignation in a meeting on Monday.

That was the same day that top executives met with representatives from Jana and Ontario Teachers, who presented a plan to split the company into four groups: an education business, a media and information arm, S.&P.’s ratings and financial business and its Standard & Poor’s indexes.

The investors’ rationale is that the four units share little in common operationally and have different, and sometimes conflicting, capital needs. And the high-growth operations like S.&P.’s have been hidden by the low growth at divisions like publishing and education.

Many of Jana’s recommendations are in line with the thinking of research analysts. According to Goldman Sachs, McGraw-Hill is undervalued by 20 percent compared with its peers.

McGraw-Hill was already reviewing its portfolio of businesses by the time Jana, a hedge fund based in New York, and the Ontario Teachers fund, of Canada, built up their stake to 5.6 percent. Mr. McGraw has said the company expects to make a major announcement in the second half of the year.

Azam Ahmed and Nelson D. Schwartz contributed reporting.