Sergey I. Kislyak, the longtime Russian ambassador to the United States, hosted a dazzling dinner in his three-story, Beaux-Arts mansion four blocks north of the White House to toast Michael A. McFaul just weeks before he took up his post as the American envoy to Russia.

It was, Mr. McFaul recalled, an “over-the-top, extraordinary dinner,” including five courses of Russian fusion cuisine for 50 seated guests who shared one main characteristic: They were government officials intimately involved in formulating Russia policy for the Obama administration, including senior figures from the Defense and State Departments.

“I admired the fact that he was trying to reach deep into our government to cultivate relations with all kinds of people,” Mr. McFaul said of the dinner in late 2011. “I was impressed by the way he went about that kind of socializing, the way he went about entertaining, but always with a political objective.”

Mr. Kislyak’s networking success has landed him at the center of a sprawling controversy and made him the most prominent, if politically radioactive, ambassador in Washington. Two advisers to President Trump have run into trouble for not being more candid about contacts with Mr. Kislyak: Michael T. Flynn, who was forced to resign as national security adviser, and now Attorney General Jeff Sessions, who admitted two previously undisclosed conversations. Mr. Kislyak also met during the transition with Mr. Trump’s son-in-law and adviser, Jared Kushner.