The downgrade of US debt to "AA+" was criticised by the Obama administration. Officials claimed that the agency had gone ahead with the move despite admitting to a US$2 trillion ($2.4 trillion) "error" in an early version of its decision.
The downgrade, which came after a rancorous political battle to raise the US debt ceiling, triggered stock market falls.
However, it was not matched by the other large ratings agencies Fitch and Moody's.
Moreover, in a sign that investors still view the country's debt as a safe haven, US Treasuries have attracted interest throughout the recent market turmoil, with yields falling to record lows.
Peterson will take over on September 12, with McGraw-Hill, which has come under pressure from shareholders to restructure, saying that Sharma will stay on until the end of the year to work on a strategic review. Shareholders in S&P's parent have been calling for a break-up of the group, which also owns the textbook publisher McGraw-Hill Education.
In a Securities and Exchange Commission filing this week, the hedge fund Jana Partners and the Ontario Teachers Pension Fund, which together own 5.6 per cent of McGraw-Hill, also called on the company to "bolster" the ratings service with an "independent oversight figure".
- Independent