Ten years ago, Tinsley Mortimer was the most photographed socialite in New York City — complete with her own reality TV show, signature lipstick shade and handbag line.

Now she’s living a secret, abusive nightmare in Florida.

Last Saturday, the 40-year-old Mortimer was arrested for trespassing at the Palm Beach, Fla., residence of her on-again, off-again boyfriend, Alexander “Nico” Fanjul, 30, the son of sugar baron Alexander Fanjul.

According to the police report, Fanjul called cops when Mortimer showed up uninvited — while Fanjul’s new girlfriend was there. When authorities arrived, Mortimer was “crying and screaming irrationally,” saying her purse was inside.

The socialite — who in 2008 launched her very own shade of Dior lipstick, Tinsley Pink — was booked into the Palm Beach County jail, charged with trespassing, and spent nearly three hours in the slammer.

Now the blond beauty, who went from diamond cuffs to handcuffs in less than a decade, has morphed into a cautionary tale for Pretty Young Things. What went so wrong?

For one thing, her 10-years-junior boyfriend, Fanjul. His billionaire family owns the right land, belongs to the right clubs — like the famously snobbish Bath and Tennis Club in Florida — and is a fixture on the party pages of The Shiny Sheet, the nickname for the Palm Beach Daily News. According to a source, Fanjul has been studying for his insurance license exam for the last three years.

He and Mortimer started seeing each other in December 2012. But Fanjul kept the relationship clandestine and mainly confined to after-hours hookups. Numerous people interviewed for this story called the duo’s dating life “unhealthy.”

But few knew just how unhealthy.

Since December 2013, there has been one arrest, four police reports and a written warning from the Palm Beach Police Department — all of which document physical abuse and trespassing. Sources say the ordeal has been a living hell for Mortimer, who continued to let Fanjul back into her life, hoping he would change and fearful of the havoc his family might wreak should she go public with claims of abuse.

According to one police report, Mortimer was hospitalized “due to a possible battery” on Christmas Day 2013.

“We saw that Tinsley had a laceration on the back of her head that was still bleeding . . . The left side of her face next to her eye was very swollen with yellow-and-green bruising. We also saw red and bruised markings on her forearms, biceps and elbows,” wrote the officer after visiting Mortimer, who had her mother, Dale Mercer, by her side, at the Good Samaritan Medical Center in West Palm Beach.

She received three staples in the back of her head. Originally, Mortimer told officers that she tripped and fell.

Days later, on New Year’s Eve, Fanjul called the police because of an “unwanted guest.” The cops arrived to find Mortimer locked in a bedroom at Fanjul’s home declaring she was fearful of him and wanted to prosecute her sometime lover. The police report says she “appeared intoxicated” and her speech was slurred. She was issued a written warning for a trespass.

On New Year’s Day, she came clean in a phone conversation with police. According to that report, Mortimer was trying to intervene in a fight between her boyfriend and his younger brother, Oliver “Reddy” Fanjul, when she was “struck in the eye by Reddy.” Nico then “defended his brother,” and “the argument escalated to the point he covered her mouth and nose,” the report says.

It also read that Mortimer “made it clear she does not want to prosecute for the battery. She made several references to how powerful the family is” and his “family’s status in Palm Beach.”

Still, Mortimer continued to date Fanjul. A source says she was drawn back in by his apologies. (Calls to Fanjul were not returned.)

“Love is an addiction. You lose yourself,” says a friend of Mortimer’s. “I think [the arrest] is a good learning experience for her, as horrible as it is, to move on.”

Once upon a time (in the mid-2000s), Mortimer was the Queen Bee of New York City society. She was wed to her high school sweetheart, Topper Mortimer, whose great-grandfather was a director of Standard Oil. She graced the pages of Vogue (where she had previously worked as a beauty assistant), made a cameo on “Gossip Girl,” commandeered Fashion Week front rows, and managed to parlay her broad smile and bouncy curls into serious business deals. Among them: a purse line with Japanese company Samantha Thavasa and a highly coveted gig as Dior’s beauty ambassador.

“She was set up basically to be the next Cornelia Guest, the next It girl,” says one New York City society publicist.

But the life of the Virginia native took a downturn when she and Topper separated in 2009 amid allegations that she was too enamored with fame — and her illicit lover, Casimir “Cassi” Wittgenstein-Sayn, a German royal.

“After her marriage dissolved, she became another blonde in a room instead of Mrs. Topper Mortimer,” says the publicist.

In 2010, Mortimer’s reality show, “High Society,” premiered on the CW network to sneers and gasps. In one scene, her co-star Jules Kirby admits that the N-word “really should be OK to say.”

“What killed [Mortimer] was that reality show, honestly,” says someone who has sat on charity boards with Mortimer. “That was her demise.”

Since separating from her husband in 2009 and divorcing in 2013, Mortimer has been linked to men including “Bachelor” star Prince Lorenzo Borghese and “American Idol” alum Constantine Maroulis, who tells The Post that Mortimer’s “got some growing up to do” but is a “lovely girl.”

The former party-photo staple tells The Post that she moved to Florida in 2012 “to get away” from the pressures of the spotlight, and to be close to her mom.

But, as New York Social Diary creator David Patrick Columbia points out: “The problem with Palm Beach is that it’s too much money, and if you don’t have it, you’re nobody. And that’s tricky for a pretty girl like Tinsley.”

Mortimer — who grew up well, but not with the billions of her new neighbors — tried to adapt to life out of the public eye, ditching her Oscar de la Renta gowns and black-tie balls for a day job planning events for a tax company called Engineered Tax Services.

But drama found her when she met Fanjul.

‘She needs to love herself more, and respect herself more, and find someone who’s going to care about her.’ - Constantine Maroulis, Mortimer's ex

In December 2012, they embarked on an often-secretive relationship, which only ended earlier this month, according to a source. The two stuck to dates at less sceney restaurants in West Palm Beach so as to not be spotted by his disapproving family, who allegedly thought Mortimer was too old and not good enough for Fanjul, and her friends, who had heard of their altercations and wanted her to cut ties.

When news of Mortimer’s arrest hit the press this month, many were quick to assume she had gone crazy. “She lost her head over one of Palm Beach’s richest, most eligible men and she doesn’t understand that the dance is over,” quips the society publicist.

But few knew what was really going on behind closed doors.

In June 2014, police were called to Fanjul’s $1.6 million home for an “unwanted guest call.”

When cops arrived, they found a barefoot, intoxicated, crying Mortimer next to her mother’s Land Rover, which she had driven that evening. She told police that she and Fanjul had arrived at his place after dinner at a restaurant. They got into an argument about a boat trip Mortimer had taken with friends weeks earlier.

“[Mortimer] said Fanjul . . . tried to smother her with a pillow,” according to police. She fought back, ran outside, and locked herself in the car.

Fanjul chased her, keyed the car, ripped off the windshield wipers and used them to break the windshield, the police report says. Mortimer took an iPhone video of the entire incident.

When officers entered the bedroom where the original altercation took place, “there were blood marks on one of the pillows,” they reported. A “highly intoxicated and uncooperative” Fanjul asked cops to leave. Mortimer refused to sign a sworn affidavit of prosecution.

In December of the same year, police were dispatched to Fanjul’s house regarding “a suspicious incident involving a male and female involved in an altercation.”

Mortimer had fled by the time they arrived, the report says, but a drunken Fanjul was there, with scratches on his chest and back and bruising above his rib cage.

Fanjul’s neighbor Richard Spehr told police he “witnessed Fanjul tackle [Mortimer] and push her head into the pavement.” Later, Spehr said Fanjul spoke to him and “apologized that they had to see that.” (Spehr declined to speak to The Post.)

Police made contact with Mortimer nearly two weeks later. Despite admitting that this wasn’t the first time Fanjul had attacked her, and that she had scratched him in self-defense, she did not wish to prosecute.

Reached by The Post in Palm Beach, Fanjul’s mother, Nicole Fanjul, denied the allegations of her son’s abuse.

“It’s all hearsay, and all [Mortimer’s] black-and-blue stuff is what she’s done to herself or what somebody else has done to her,” Nicole says. “She hides in the bushes, she hides in her car, she puts cigarettes out in his face, did anybody ever tell you that? This makes ‘Fatal Attraction’ look like a cartoon. She’s insane. This is a result of a woman scorned.

“She’s going up against us and she’s going down.”

Text messages between Mortimer and Fanjul obtained by The Post reveal “I love yous” being exchanged as recently as April 3, after a fight over his fidelity.

“Both are great people, but together they are not great,” says Mortimer’s pal. “They’re combustible. They use the police to manage their relationship.”

No doubt, this wasn’t how things were supposed to end up. Mortimer grew up in Richmond, Va., with her father, George, a rug salesman-turned-real estate investor; mother, Dale, an interior designer; and younger sister, Dabney. Her father, who passed away last year, bragged in a 2006 interview: “We’re in the [Social] Register. Her ancestor is Thomas Jefferson. She was raised in the biggest home in central Virginia. We had a nanny, two gardeners and a butler.”

In 2012, Mortimer wrote a novel, “Southern Charm,” and about a year and a half ago, she launched a line of home goods manufactured by Pop Culture Living, the same company that produces tabletop items by Flo Rida and Lisa Vanderpump. According to Pop Culture Living President Rob Berman, the Four Seasons Las Vegas and the Boca Raton Resort and Club use Mortimer’s lacquered, preppy accessories as décor.

Mortimer — who, a source says, has frozen her eggs — hasn’t given up on the dream of having it all: a family and Tory Burch-level success.

“I don’t regret any of the decisions made,” says Mortimer of her failed reality show and move to Palm Beach. “The one regret I have is that I stayed in an unhealthy relationship for as long as I did.” She refused to elaborate.

Maroulis has heartfelt advice for his ex: “She needs to love herself more, and respect herself more, and find someone who’s going to care about her.”

If you or anyone you know is a victim of domestic violence, call the National Domestic Violence Hotline for help at 800-799-SAFE (7233).