Wealthy developer Steve Witkoff had just moved his offices to West 57th Street — Manhattan’s Billionaires’ Row — when he first laid eyes on Lasharn Francis Harvey and her sand-colored dog, Sahara.

Lasharn had set up a sleeping bag outside the massive granite facade, and would lay her head on the cold sidewalk every night.

In the morning, Lasharn would routinely call out, “Have a blessed day!” as he entered his building.

Steve knew his life was blessed. He develops millions of square feet of offices and condos in New York, Miami and London. But he also felt cursed, by heartbreak, since the death of his 22-year-old son, Andrew, five years ago from an Oxycontin overdose.

Maybe there was a way, he thought, to help Lasharn. He didn’t realize she could help him, too.

Lasharn was born 43 years ago in Chicago, and, for a while, life was good. She went to college and aspired to run her own business. She sold a co-op in 2008, and invested the $140,000 profit in a technology she saw as a sure thing.

Ever heard of DVD vending kiosks? Rent a DVD on a street corner for $1 a day, secured by a credit card? Lasharn bought two of the doomed things. Then the recession hit. Her little business foundered.

She had bought the kiosks for $60,000, only to sell them for around $16,000, she said.

“Yes, it was devastating,” she told The Post. “But you just pick up your feet and start again.”

She decided to travel.

“I sold my apartment and bought a dog,” she said.

Sahara is part golden retriever and part poodle. Lasharn paid $800 for the scruffy thing, all curls and love, and off they drove to see America.

Lasharn is hesitant on the specifics, but her travels turned out to be a series of wrong turns, including in Florida, where she enrolled in a school to become a “traveling physical therapist.” Nothing came of that, either.

Maybe it was bad luck, or her own regrettable business decisions, or the damage from the tumor she didn’t realize was growing slowly in the pituitary gland at the base of her brain. But several years ago, Lasharn found herself with nothing but Sahara. Not that anything ever got her down.

“It’s easy to be positive when you are homeless,” Lasharn said. “People give you money and try to help you.”

Arriving in New York, she could have set down her cup of loose change and her sleeping bag at Port Authority, or at Penn Station. Instead, she and Sahara decided to live outside 40 W. 57th St.

“You couldn’t help but notice her,” Steve said. “Every single morning she was there, sitting on the ground with Sahara, whether it was cold or wet. She was there in the rain and she had a small cup with change, and that wasn’t going to be sufficient. And that is how I met her.”

Steve remembers their first meeting. “Sahara jumped on me and knocked the change out of the cup” as he made the first of many donations.

“I see a lot of homeless people,” said the 59-year-old mogul. “But she had a kind face. You could tell that she was kind and that she was kind to Sahara. Even before I gave her the money, I talked to her about Sahara and she said Sahara was her baby.”

Since Andrew’s passing, Steve Witkoff has become a quietly generous man, often slipping cash into the cups of beggars he passes on his walks, meeting to meeting, through Midtown.

Still, there was something special about Lasharn, he said — with her reliable, all-weather cheer and her contented dog.

“I have dogs,” added Steve, who, in fact, has seven. “And I knew that if Sahara was happy and incredible, then there had to be something nice about Lasharn.”

“And then,” added Steve, “I remember that Sahara was matted, and I was concerned. She was living on the street and I worried that she needed medical care and I said I would have her groomed,” he said. “But Lasharn thought I’d take the dog.”

It took days to build trust. Steve persisted, introducing Lasharn to Samantha Schmidt, his “dog whisperer,” the woman who managed the care for his own small pack.

“Finally I convinced her, and we sent [Sahara] to the grooming place on Lexington and then Sam took Sahara with Lasharn to my vet and Sahara was checked. But they were still living on the street.”

Weeks later, on Thanksgiving, Lasharn and Sahara were invited to join Steve’s family and friends for a big dinner at the Park Lane, a luxury hotel on Central Park.

“Seventy people stuffed into the apartment, and she came with Sahara,” he remembered. “And Sahara sat next to her and ate turkey and stuffing.”

That was a place of honor — Steve’s own dogs were relegated to the canine equivalent of the little kids’ table: dinner with a dog sitter in another room.

Then November turned to December. “It was freezing out,” Steve remembered.

The developer made sure she kept a phone on her — and his number. “I would drive on 57th Street and just make sure she was OK, because she moved around a bit.”

For Christmas, he checked Lasharn and Sahara into the Park Lane hotel, and sent them there again, over Lasharn’s protests, during a 10-day cold spell in January.

Lasharn told her benefactor that she was saving the money he was giving her, rather than spending it on housing, so she could buy a van to live in.

“I asked her how much it would be. She said $1,500.” Done. Lasharn used her new white van as transportation and housing, parking it at night in a lot on 54th Street.

“I got nervous when I didn’t see her in front of the building,” Steve admits.

But, unknown to Lasharn or her patron, the tumor at the base of her brain was growing.

She began losing her eyesight. In May, she fell, wound up in Roosevelt Hospital, and lost everything all over again. Her beloved Sahara was taken by animal control. The van would be stolen. Even her cellphone, left in the van, was gone.

“Lasharn went into the hospital with nothing, not even her phone,” Steve said. “She remembered my office number and that’s how the hospital contacted me. I was her contact.”

Steve tracked down Sahara, sending Schmidt, his “dog whisperer,” to reclaim her from the pound.

The mogul or his business assistants remained with Lasharn through the CAT scans and biopsies. Steve met with Lasharn and her doctor as she planned the radiation and steroid treatment that will shrink the tumor, and got his childhood best friend, David Cooper, a health care executive, to find an expert to double-check her reports.

With steroids, Lasharn’s tumor is shrinking, and her vision is beginning to come back in one eye.

“A part of me feels that Andrew is leading me to people like this,” Steve said.

“If she is left on the street, she will die. She is a human being and she was scared for her life. I wanted to be at the hospital to comfort her. She had no one here with her. It’s what you would do for a family member, but she didn’t have one and as each thing came up, we said we weren’t going to stop there.”

Steve now is paying for Lasharn to live at a Marriott long-term-stay hotel in Midtown while she continues her outpatient treatments. Sahara, the once-homeless dog who brought them together, and who was almost lost forever to the shelter system, is now living with one of Schmidt’s training staff elsewhere in Manhattan. Steve is paying for Sahara to learn to become Lasharn’s service and seeing eye dog.

“She would have been lost without that dog,” Steve said.

“He is such a good guy,” Lasharn said of her multimillionaire savior. “He is the best guy ever.”

But Steve insisted, “This story isn’t about me. She touched me, and I am happy to have done what I did.”

The developer has often wondered how Lasharn, who is so clearly engaging, charming and intelligent, could wind up homeless. “She went to college and she owned a co-op,” he said.

“There is a whole life here. So what happened? It is the elephant in the room … God knows how long she was living with the growth inside her brain.”

“She could have died. But this woman is a portrait in courage. She is really an incredibly courageous woman. She was all alone on the streets of New York — and still, the voice message on her phone says, ‘Have a blessed day.’”