"We as diplomats have to be at the forefront of trying to solve problems. You don't want to have a war, you want to avoid war, which means diplomats need to be able to talk," Grenell said.
"If you want to solve problems, you better have diplomats who are really tough -- diplomats who know how to push and know how to cajole ... so, I like having a president who's willing to be very tough."
He also pushed back on the idea that Trump's rhetorical style wasn't effective.
"I think $400 billion in new defense promises for NATO members is one surefire way to point to the fact that the president's style has worked," the ambassador said.
Grenell's comments came after Attorney General William Barr said Trump's tweets about ongoing Justice Department investigations and prosecutions made it "impossible" for him to do his job.
"I'm not going to be bullied or influenced by anybody ... whether it’s Congress, a newspaper editorial board, or the president," Barr said in an interview with ABC News.
Trump's tweets have long been controversial among political analysts, but Grenell claimed that Americans outside Washington appreciate the president showing annoyance with foreign countries who take advantage of the U.S.
He said the grumbling by European nations over Trump's push to have them contribute more to NATO and other initiatives was to be expected.