PALM BEACH, Fla. — Minutes before the world found out that President Trump had ordered a missile strike against Syria in retaliation for a chemical attack on civilians, the president took his usual stroll through the crowd of people eating dinner at Mar-a-Lago here. When he spotted Shannon Donnelly, a journalist who has chronicled Palm Beach society for decades, he approached her table, where she was seated with other guests.

“Big night, Shannon,” a subdued-looking Mr. Trump said. “Big night.”

The exchange, which Ms. Donnelly recounted last week in The Palm Beach Daily News, was brief, but telling. Mr. Trump, who has developed a nearly symbiotic relationship with individual members of the news media, tends to make a beeline for familiar journalists in New York and Washington when he has something he wants to share.

Ms. Donnelly is that person in Palm Beach, where she has covered Mr. Trump since he was just another billionaire who decided to stake a claim to this palm-lined playground for the superwealthy when he bought Mar-a-Lago in 1985. Mr. Trump has made seven trips back since becoming president, and Ms. Donnelly has been a fly on the gilded wall during several of his visits. While other journalists who cover the president remain cordoned off, Ms. Donnelly has an unusual degree of access, filing detailed dispatches as a guest of club members or charities.

Ms. Donnelly, an editor and columnist, tends to observe her surroundings with more creative freedom than usually afforded to the White House press corps: “This place has been home to more flashes than a double-blind menopause study, and now all of a sudden they’re cracking down?” she wrote on the evening of the Syria strike, after a member of Mr. Trump’s security detail threatened to take her phone.