Patricia Cornwell showed up to her first meeting at the Richmond, Virginia, medical examiner’s office in the summer of 1984 carrying what appeared to be a cane.

The woman who would one day invent the forensic thriller was just 28 years old and still unpublished, living in a cramped seminary apartment with her then-husband while she tried to write mystery novels that no one wanted to buy. A friend had arranged an introduction to deputy chief medical examiner Marcella Fierro, and Cornwell was determined to make an impression.

“The secretary saw me walking in and said, ‘What do you have a cane for?’” Cornwell told the Post in an exclusive interview. “And I said, ‘Oh, this isn’t just any cane.’”

In the ME’s conference room, she demonstrated — putting her lips to one end of the cane to blow a dart across the room, where it buried itself in an anatomical poster on the wall. Fierro tried it herself, then sat down and delivered the verdict.

Everything Cornwell had engineered for her novel — the poison dart, the digitalis used as poison, the aluminum pipe concealing a bamboo skewer for finishing the job if the dart failed — was clever, but it wouldn’t fool a competent forensic pathologist. The puncture wound to the heart would be obvious, and a toxicology screen would detect the digitalis.

“She said, ‘You’d be caught,’” Cornwell remembered. “And then she said, ‘Let me tell you about foxglove and digitalis.’ So I met my evil twin.”

It was, as Cornwell explains in her new memoir, “True Crime” (Grand Central Publishing), out Tuesday, the moment everything changed.

She’d recently arrived in Richmond with an unfinished murder mystery about voodoo and poisons and a fictional female chief medical examiner named Kay Scarpetta. The character, a brilliant, unflappable forensic pathologist, would eventually anchor 29 novels, nearly all of them bestsellers. But in 1984, Scarpetta existed only in rough drafts that weren’t working, and Cornwell had no idea why.

Walking into the Office of the Chief Medical Examiner changed that.

“I wanted to write with authority,” Cornwell said. “When you know your stuff, you feel a sense of confidence when you set a scene or write a story. You know what the hell you’re talking about. You’ve been there. You smelled it. You’ve seen it. You heard it.”

She became a fixture at the office, and then a part-time employee. She watched autopsies, drove the “morgue wagon,” helped weigh organs and hung bloody clothing to dry. To get legal standing to be present at crime scenes, Cornwell signed up as a volunteer police officer and went through the academy.

“Who would have ever thought that going to the morgue would save my life?” she said. But that’s more or less what happened.

Cornwell had been struggling for a decade with a severe eating disorder. But it stopped, completely and without explanation, sometime during her years at the OCME.

“I couldn’t figure out why,” she said.

Her second spouse, neuroscientist Dr. Staci Gruber, offered a theory: “She said, ‘It’s because you’ve taken control of your life. You discovered something you wanted to know about. You wanted to do it so that you could write with authority,'” Cornwell recalled. “That’s what happened.”

“True Crime” arrives just as Cornwell approaches her 70th birthday. It’s a book she spent decades insisting she’d never write.

“I’d never thought of doing it,” she said. “But then when I started fiddling with it — I had a few months because I’d finished my last Scarpetta novel early — it just took over. It was electrical. I don’t even know how it happened.”

What she produced is a story that begins with a psychotic mother — her own — burning the family’s clothing in a fireplace in Montreat, North Carolina, and ends with Cornwell standing opposite Nicole Kidman on a Nashville soundstage as the actress prepared to play Kay Scarpetta for the Amazon Prime series “Scarpetta.”

It also covers the how famed evangelist Billy Graham and his family quietly took in a 9-year-old Cornwell; her Charlotte journalist job that ended in assault and near-ruin; her years in the morgue; the Jack the Ripper investigation that consumed a decade of her and millions of her dollars; and 36 years of trying, and failing, to get Scarpetta made into a TV show.

“You almost can’t make this stuff up,” Cornwell told The Post. “So many bizarre things have happened to me.”

The memoir is also, in Cornwell’s telling, a message in a bottle to the past version of herself that didn’t know if things would work out.

“When I was coming along, I would have loved to read this book,” she said. “Especially when I was a teenager, feeling so lost and hopeless. If I’d come across something like this, written by Agatha Christie or P.D. James, I would have felt so much better about my chances.”

“Postmortem,” the first Scarpetta novel, was published in 1990 and won five major crime fiction awards in a single year, sweeping the Edgar, Creasey, Anthony, Macavity and French Prix du Roman d’Aventure awards, a feat that had never been done before.

Today, the blowgun that Cornwell brought to that first visit to the OCME is framed and hanging in her office. So is a voodoo doll she made by hand while writing her first unpublished mystery, which centered on a killer using voodoo rituals in the Belgian Congo.

“I dried an apple in the oven, painted the face on with Liquid Paper, glued yarn on the head for hair,” she said. “Still have it.”

The author said she didn’t construct Kay Scarpetta so much as discover her, through all the autopsies and homicide scenes and morgue-wagon runs and late nights with Richmond detectives.

“Every new book, I learn something new about Scarpetta’s background,” she said. “What it was like when she was in law school, who her roommate was. None of this existed when I wrote ‘Postmortem.’ It’s sort of like I’m writing biographies.”

She’s been learning more about Scarpetta with each successive book for 35 years, the same way she learned about Ruth Graham, the wife of evangelist Billy Graham, while researching the 1983 biography “Ruth, a Portrait: The Story of Ruth Bell Graham.”

The two women had an unlikely bond. Cornwell had grown up in Montreat, North Carolina, the small mountain community where the Grahams lived, and Ruth had quietly taken the disheveled, fatherless girl next door under her wing, giving her rides in her Oldsmobile and inviting her up to the family’s mountaintop home. It was Ruth who first pressed a journal into a teenage Cornwell’s hands and told her to write her story.

“She wanted to do something for me, to give me a chance,” Cornwell said of Ruth.

“True Crime,” she added, reads to her like a modern fairy tale — with Ruth as its unlikely fairy godmother. But every fairy tale has its villains, too.

“There are monsters I encountered along the way,” Cornwell said, referring to a foster parent who terrorized her as a child. “You can’t tell the true story of your life without pointing out some of that.”

It’s the same instinct that drives Scarpetta. Follow the story, she said, even into the rooms you’d rather not enter.

The character, she said, is “a separate entity that keeps me company while I’m sitting in front of my computer,” Cornwell said. “Contrary to what people might think, I’m not aware of her the rest of the time. I don’t walk around thinking, what would Scarpetta say about that? I always think what I think. But when I’m in [a character’s] world, it’s like Alice going through the looking glass. I go to where they are, and they seem to exist separate from me.”

Years ago, at a book signing, a fan asked for Scarpetta to sign a copy of “Postmortem,” not Cornwell. “I said, ‘let me see if I can conjure her up.’ I’d never done this before,” she said. “The pen shifted into my left hand and I signed it left-handed. That’s when I knew that Scarpetta was left-handed. I’ve written her as left-handed ever since.”

At the premiere of the “Scarpetta” television series, the project Cornwell spent 36 years convinced would never actually happen, Kidman walked into the room and something unexpected happened. “I had this funny little feeling inside of me, as if Scarpetta had just walked in,” Cornwell recalled. “I thought, ‘Am I meeting my character?’”

The author filmed a cameo in the series, as a judge swearing Scarpetta in as Virginia’s chief medical examiner, and Cornwell said her knees shook while on camera. This is a woman who’s watched hundreds of autopsies, ridden with homicide detectives through the worst nights in Richmond, and walked Whitechapel with Scotland Yard’s most senior detective discussing Jack the Ripper murders, while remaining unflappable — no shaky knees.

“I completely forgot what I was supposed to say,” Cornwell remembered with a booming laugh. “I looked at everybody and I said, ‘What is it she does for a living?’”

The crew laughed, and they did another take. This time she got through it. When it was over, Cornwell leaned in and whispered to Kidman, “I really swore you in. I wasn’t kidding.”

Hours later, her phone lit up. Charlie Cornwell, her ex-husband, the seminary student who’d accidentally given Scarpetta her name when he mentioned a landlady at the University of Virginia, had died after a three-year battle with lung cancer.

She’d been to see him in September, a few weeks before filming. He was 85 and frail, and he knew the end was near. They’d talked about what comes next, and Cornwell told him what she’d come to believe from all those years in rooms with the dead.

“I knew the body on the table wasn’t the person,” she writes. “What we leave behind is like a discarded old shoe. Death isn’t the end because we aren’t these bodies. We just live in them.”

Charlie’s voice was a whisper. He asked if there was anyone she wanted him to look for on the other side.

“Ruth,” she said without hesitation, meaning Ruth Graham.

He agreed he would.