On a premier block of Manhattan’s Upper East Side, just one door down and across Fifth Avenue from Central Park, a strikingly modern residence tops an imposing six-story limestone mansion designed by noted Gilded Age architect Charles Pierrepont Henry Gilbert.

Known for the opulent townhouses and manses he designed for robber barons and Fifth Avenue socialites, C.P.H. Gilbert created a Beaux-Arts palace of sorts at 3 East 75th Street, which he designed in 1904 for Stuart Duncan. His father, John Duncan, was a high-end grocer, with a store at 1 Union Square and a fortune made as the sole U.S. distributor of what was then a new and exotic British condiment: Lea & Perrins Worcestershire sauce.

That lucrative importing venture funded quite the home, with signature touches like a grand marble staircase, a 50-foot-wide ballroom with 20-foot ceilings and a squash court with a spectators’ gallery on one of the upper floors. To soundproof the interior, some of the walls were two feet thick, and rooms were paneled in exotic woods.

In 1920, Duncan sold 3 East 75th to Clarence H. Mackay, head of the Postal Telegraph Cable Company (later ITT), which handled trans-Atlantic communications. Mackay’s daughter, Ellin, was living there in 1924 when she met and fell in love the composer Irving Berlin, whose songs include ”God Bless America” and ”White Christmas.” Berlin was Jewish, and Mackay, a devout Roman Catholic, opposed the marriage, even sending Ellin on a voyage to Europe to discourage the relationship. But on Jan. 4, 1926, Berlin telephoned Ellin Mackay at the house and proposed immediate marriage. She walked out of 3 East 75th in the clothes she was wearing, took her first subway ride and married Berlin at the Municipal Building, according to ”Irving Berlin: A Daughter’s Memoir” (Simon & Schuster, 1994) by their daughter, Mary Ellin Barrett.

In 1941, after Mackay’s death, the grand 50-foot-wide mansion on one of the quietest blocks in Lenox Hill was divided into apartments for 23 families. Today, it holds seven luxury condos, with the sprawling triplex penthouse on the top floors as the crown jewel of the Gold Coast building, the Stuart Duncan House.

Designed by Thad Hayes and offered in triple mint condition, the 7,325-square-foot apartment features three bedrooms, three full bathrooms, two powder rooms and an astounding 3,600 square feet of usable outdoor space.

The middle level is the main floor, with a large open-plan great room with loads of flexibility that is now set up with living and dining spaces. It features hardwood floors, high ceilings, oversized windows, and art-friendly open walls. The sleek and modern galley-style kitchen offers state-of-the-art appliances, and a walk-in wine room. A quite large terrace, which is accessible from both the great room and the kitchen, offers a pergola and a Viking outdoor grill. The flow is ideal and it’s perfect for entertaining.

Take the stairs up to the top level, which holds a large screening room that connects to a game room with a kitchenette and plenty of room for a billiards table. Outside, the second level of the terrace offers gorgeous views across Central Park.

The cozy lower level holds a gallery, a home office/den and the three bedrooms, including the primary suite. It features a terrace, south-facing windows, and a large walk-in closet. Two more small terraces and a service kitchen round out this floor.

Building amenities add a live-in super, a part-time doorman and laundry facilities.