The afternoon event started with stunning Connecticut coastline views, crisp G&Ts, and Aristotle and ended with the mayor of New York arriving via boat and toasting the couple with good cheer and Italian sandwiches. In between, there were dozens of well-wishers celebrating the engagement of New Yorkers Penelope Diamandis and Chase Ellsworth Blakeman at the Fair Harbor home of Mr. Blakeman’s parents, George and Abigail Blakeman. The wedding is planned for late December in Montecito, California, where Ms. Diamandis was raised by her mother, Alexa Diamandis.

The couple met the old-fashioned way—on the corner of Seventy-Sixth Street and Third Avenue in Manhattan. Both early morning runners in Central Park, they had been eyeing each other for months. Mr. Blakeman, a graduate of Fordham University, is the chief of staff for the mayor of New York City, Timothy Lynch. Long hours by the mayor’s side meant that his workout time often came before sunrise. Ms. Diamandis, a graduate of Southern Methodist University in Dallas and now a senior account executive at Lynx Communications, a boutique public relations firm specializing in travel, is an early riser as well. The two would often find themselves headed back to their respective Upper East Side apartments at the same time after a loop around the reservoir. One morning, before ducking into a deli for coffee, Mr. Blakeman introduced himself and asked for Ms. Diamandis’s number so he could ask her out.

Instead, she gave him her work email and little hope. “He always wore the same T-shirt when he was running. I was worried he didn’t know how to do laundry,” Ms. Diamandis recalls.

It turns out that Mr. Blakeman had more than one college T-shirt, and his persistence won over the California native. Over the next eleven months, Ms. Diamandis learned the city outside Manhattan while accompanying Mr. Blakeman on the weekends to community events and fairs in the outer boroughs. The two shared subway rides, churros, karaoke contests, and bonding with constituents. “We were a good fit from the start,” Mr. Blakeman says. “Penny jumped right into cakewalks and barbecues and whatever might be happening in Queens or Brooklyn or the Bronx. I fell in love with her optimism.”

As for Ms. Diamandis, who spent her childhood summers on the island of Patmos in the Aegean Sea and traveled extensively throughout Europe with her mother, who is a travel professional, she said, “With Chase as my tour guide, New York City became my oyster.”

Thanks to some help from his boss, Mr. Blakeman dropped to one knee at the top of the Empire State Building while on a prearranged private tour, offering Ms. Diamandis an aquamarine and tourmaline ring from designer Evvie Taylor. Irish singer-songwriter Dermot Kennedy, a supporter of the mayor, serenaded the duo with his hit single “Kiss Me” while photographer Colin Brookes snapped photos of the newly engaged couple. Afterward, a table at Bemelmans Bar was waiting for them.