As ambassador, Mr. Johnson has had to navigate Mr. Trump’s up-and-down relations with British leaders. The president soured on the prime minister at the time, Theresa May, and berated her on trans-Atlantic phone calls. His relations with Prime Minister Boris Johnson, a like-minded populist, have been warmer, though Mr. Johnson has sometimes steered clear of Mr. Trump, who is deeply unpopular in Britain.
A prominent Republican donor, Ambassador Johnson initially supported Jeb Bush for the Republican nomination in 2016, but he later backed Mr. Trump, introducing him to other figures in the party’s money circles. Enlisting Mr. Johnson as an emissary on behalf of his golf course was another way the president was looking for help furthering his financial interests.
Beyond the legal and ethical red flags, asking for such a favor from his host country would put Mr. Johnson in an untenable position as the emissary of the United States.
“It is diplomatic malpractice because once you do that, you put yourself in a compromised position,” said Norman L. Eisen, who served as President Barack Obama’s special counsel for ethics and later as his ambassador to the Czech Republic. “They can always say, ‘Remember that time when you made that suggestion.’ No experienced diplomat would do that.”
For Mr. Johnson, 73, London was a reward fit for the billionaire heir to the Johnson & Johnson pharmaceutical fortune. Formally known as the Court of St. James’s, the assignment is the plum of the diplomatic corps — one that comes with a palatial residence, Winfield House, and entree to the highest levels of British society.
Like many political appointees, Mr. Johnson had no diplomatic experience before arriving in London. Affable and well connected, he is known mainly for the nickname Woody and his ownership of the New York Jets, a perennially struggling N.F.L. franchise. His transition to leading a large embassy was bumpy.
Mr. Johnson’s throwback style has been criticized as offensive. There have been complaints that he complimented the appearances of female employees during staff meetings, and after interviewing a candidate to replace Mr. Lukens as deputy chief of mission, he asked a colleague whether she was Jewish.