McCarthy, the majority whip, will serve as Romneys statewide chairman.
After a long and grueling primary, McCarthy said in a statement, it is clear that Mitt Romney is the best candidate to face President Obama and fix the mess of his one and only term.
Republicans need to unite and work together if we plan to take back the White House and put in place policies that will get our nation back on a path to prosperity by reducing taxes, shrinking government, and empowering the private sector, he said.
Romney called McCarthy a leading voice in the effort to cut spending, reduce burdensome regulations, and make government more accountable to the taxpayers.
McCarthy's congressional district includes most of Kern and San Luis Obispo counties and part of Los Angeles County.