Yet there is little sign Merkel can get Trump to back down from decisions he sees as part of his populist promise to help American workers and secure better global deals.
Merkel's visit comes just after French President Emmanuel Macron was in Washington, calling on the United States not to abandon the nuclear agreement.
The stylistic differences between the Macron and Merkel visits are "very telling" according to Mona Krewel of Cornell University.
Aside from trade, the future of the Iran nuclear agreement is another subject which Merkel will raise in Washington.
The Iran deal, looming United States tariffs on European steel and aluminium products, a planned Russian gas pipeline running under the Baltic Sea to Germany, and Berlin's military spending are issues that divide Merkel and Trump and likely to come up at their working lunch. So she too hopes to encourage Trump to keep it in place.
Grenell's nomination was stalled for months - Trump nominated him in September - and the White House repeatedly cited it as an example of Democratic obstruction in the confirmation of the president's nominees.
"The chancellor is pleased about this appointment because she is a convinced Atlanticist", a senior German official said.
"Will he insult, via his Twitter account, the female chancellor of Germany?"
Germany expects Washington to impose punitive tariffs on European Union aluminium and steel imports from May 1, a government source said Thursday, hours before Chancellor Angela Merkel was to head for the United States. "And then we will see how we will handle it".
Among Merkel's priorities during her visit to Washington would be to deepen economic ties with the USA, the German government source said, adding that Berlin is ready to renegotiate industrial tariffs with the US.
Germany and Russian Federation have both rejected suggestions by Trump that the existing agreement could be renegotiated.
The next looming deadline is May 12, the date by which Trump has threatened to tear up the landmark 2015 agreement to stop Iran from developing nuclear weapons in return for relief from worldwide sanctions.
Macron came away from his meetings speculating that Trump would kill the deal, but debate continues to rage.
As head of the CIA, for instance, he had relayed the spy agency's institutional assessment that Iran was broadly complying with the terms of the agreement, despite Trump's public pronouncements.