The TV host and his wife, Nancy Juvonen, pieced together four Gramercy Park apartments over a decade to build an eclectic space filled with nooks and crannies, cubbyholes and secret passageways

Jimmy Fallon bought his first apartment, a one-bedroom bachelor pad in a historic co-op building on the east side of Manhattan’s Gramercy Park, for $850,000 in 2002. Nearly two decades later, he and his wife, Nancy Juvonen, are putting their six-bedroom penthouse, which spans three floors of the building, on the market for $15 million.

The eclectic, colorful apartment explodes with personality. Pieced together over more than a decade from combining four units, the roughly 5,000-square-foot space is filled with nooks and crannies, cubbyholes, narrow corridors and secret passageways. Nearly every wall, including the insides of cupboards and closets, is covered in art, from paintings of the couple’s favorite people to vintage wallpaper from the 1920s through the 1960s.

“I always like a secret bookshelf or a secret room,” the late-night talk show host said in an interview with his wife. “Like from ‘The Hardy Boys’ or something.”

On the property’s main floor, the couple has a large central entertainment space they call the saloon room that has vintage shelving—including custom inlaid stained glass—and a stone hearth. Decorated in tartan wallpaper, a large antler chandelier and deep coffered wood ceilings, the room has a corner seating nook with a domed top designed to look like the constellations.

Ms. Juvonen, 53 years old, a movie producer with a passion for design, has placed novelty objects and treasures from the couple’s adventures all over the property. “I don’t like part of the room to be done and the other part you’re not supposed to really look at. I like layers and layers and layers,” she said. “I feel like there is energy in these inanimate objects.”

At one point, she said, she bought so much wallpaper from a store on 23rd Street that sells vintage, Victorian and antique-patterned wallpaper that she almost exhausted their supply.

For the walls along the stairs, the couple commissioned an artist to design a mural depicting many of the family’s favorite spots, from Gramercy Park to their home in Sagaponack, N.Y., and Mr. Fallon’s office building at 30 Rockefeller Plaza. They had the artist add tiny tableaus of their family and friends, skating on the ice rink at Rockefeller Center or playing on the swings in the garden.

For their two young daughters, Frances and Winnie, the couple got especially creative. From a small hatch in the pantry, as well as from a hidden door under the stairs, the kids can access a secret playroom equipped with monkey bars, a chalkboard wall and a “jailhouse intercom” that connects to a phone in a tiny upstairs diorama room. A small fireplace in the playroom dates to the 1800s and is original to the building.

Ms. Juvonen said the playroom actually represents a scaling back of her aspirations. Originally, she had hoped to install a kind of hamster tube that would allow the girls to slide between the floors, or a periscope that would allow them to see one another. The contractors said it was impossible.

“Everyone was just so over me by that point,” she said with a laugh.

The open kitchen and dining room has bar seating and an original fireplace mantel framed with vintage orange and blue tiles. A portrait of Adam Sandler by artist Colleen Barry hangs by the window.

When Mr. Fallon, now 46, bought his first apartment in the building, he was nearing the end of his long run on “Saturday Night Live” and had been dabbling in the movie business. His boss, SNL producer Lorne Michaels, urged him to buy a place, even though he could barely afford it, and Gramercy Park was the obvious choice for him. Mr. Fallon said he had long loved the area and enjoyed watching well-dressed people spill out of the Gramercy Park Hotel and the storied National Arts Club building across the way. The fact that you needed a key to enter the park held a certain mystique for him. (The apartment came with one.)

The building itself, a redbrick property built in 1883, was also a subject of fascination. It had once been home to the actors James Cagney and Margaret Hamilton. Ms. Hamilton, who played the Wicked Witch of the West in “The Wizard of Oz,” was known to open the door to trick-or-treaters at Halloween dressed in her full costume.

“Can you imagine the door opens and it’s the actual Wicked Witch of the West? It’s so funny. It’s brilliant,” Mr. Fallon said.

As a single man, Mr. Fallon had been convinced by a friend to hire an interior designer to decorate the space, which his wife later compared to an old man’s apartment, with dark drapes and lots of maroon.

It was as if Jane Eyre had a bachelor pad, Mr. Fallon joked, noting that the kitchen was more like “an airplane bathroom they pretended was a kitchen.”

After they got married, in 2007, he and Ms. Juvonen began adding contiguous units when neighbors departed the building, and she took over the decorating. They bought their most recent unit, on the building’s top floor, for $725,000 in 2014, bringing their total spend for five apartments to roughly $5.8 million. They plan to keep one small unit at the building.

Later, they found out they had unknowingly outbid their longtime pal, author and humorist Fran Lebowitz, on one of the apartments. “She brings it up all the time. She says ‘How is my apartment?’ ” Mr. Fallon said. “When we named our daughter Fran, she was like, ‘What else do you want from me?’ ”

Mr. Fallon joked that he’d happily let Ms. Lebowitz move in with the family. “Just what she wants,” his wife added, “roommates.”

As they expanded, the couple restored window frames and casings, hardwood floors and fireplaces. They even convinced the contractors to retrofit one of the building’s old turrets that runs along the edge of all three floors of the apartment. On the top floor, they built out the turret to look like the inside of an old tree. Inside, they put a napping area from which they can look up and see the branches of the tree and twinkling lights.

“My design rule is that as soon as someone tells me what something is supposed to be, I start going in the opposite direction,” Ms. Juvonen said. “Like, oh really, that has to be a corner? OK then, I’m going to blast it out and make a bubble wall. I’m really not afraid. I have a tool belt, I have a hammer, I have every nail accessory known to man.”

The couple said they are selling the apartment because after months spent at their house in Sagaponack, N.Y., during the pandemic, it makes sense to make the suburbs their base and for Mr. Fallon to commute to work in the city. They are just the latest New York family to make a lifestyle change amid the Covid-19 crisis. On a recent walk around the neighborhood, the main entrance to the Gramercy Park Hotel was dark, and wooden huts lined the streets for chilly outdoor dining.

Listing agents Jeremy V. Stein and Debbie Korb of Sotheby’s International Realty said the home is specifically tailored to the couple’s needs but could be reconfigured to a buyer’s prerogative. The New York market, while hit hard by the pandemic, appears to be in the early stages of recovery and has posted strong contract activity over the past few weeks, Mr. Stein said.

Despite the move, Mr. Fallon said he believes in the city’s recovery. “I’m a lifelong New Yorker; you’ll still see me at restaurants, Knicks games and Broadway shows,” he said. “My job is here, my studio is here and a piece of my heart is here. My life will still revolve around this city.”

Corrections & Amplifications
The portrait of Adam Sandler in the Fallon home was painted by artist Colleen Barry. An earlier version of this article incorrectly said it was painted by actress Drew Barrymore. (Corrected on March 10, 2021.)