With her decorously styled navy blue dress, single strand of pearls at the throat and long, thick hair disciplined into an upswept bun, Mary Higgins Clark could pass as an efficient office manager or perhaps the headmistress of a fashionable girls' school.

She doesn't look the sort to scare millions of readers and hold them in chilling terror as they turn the pages of her best-selling suspense novels, but her last two alone brought her at least $1.5 million each.

She does it without demons, disembodied voices or covens of witches. Hers is a terror lurking beneath the surface of everyday life. Clark writes about ordinary people suddenly caught up in frightening situations as they ride a bus or vacuum the living room.

"I get all my terror out of the typewriter," said Clark, during a recent visit to Washington.

She is a quietly attractive woman, who would seem in place residing over the silver teapot at an afternoon reception -- if it were not for the diabolical gleam in her eye when she talks about murder trials that she has attended or headline cases in newspapers.

"Over the news broadcast in Chicago, I heard about this man who attacked a woman. He had been in the attic for a month listening to the man and wife talking in the bedroom, and repeated what they said as he beat her. Imagine the terror of that -- a man who comes in and out of the attic and eavesdrops. Such a marvelous idea!"

And Clark's eyes light up at the terror that can come to an ordinary person in his own home. It reminds one that it was a very proper English gentlewoman who concocted those devilishly cunning murders in the Agatha Christie mysteries.

"I write for the mainstream. I write about nice people not looking for trouble. They find evil in their own car, home, everyday life," Clark explains.

Sixteen years ago, at the age of 34, Mary Higgins Clark was widowed when her first husband, Warren, died of a heart attack after 15 years of marriage. Up to then she had dabbled at writing.

"Warren told me: 'Some women bowl. You write.' He wasn't being deprecating. He just didn't want me tobe disappointed by taking writing too seriously," Clark remembers.

She sold her first story to Extension, a small magazine, for what she remembers as "the magnificent sum" of $100. Her latest novel, just published as "The Cradle Will Fall," brought her $550,000 as an advance with an additional $1 million for paperback rights plus an escalator clause. It has been purchased for a major movie.

This follows the success of her two earlier suspense novels. "A Stranger is Watching" ($500,000 advance, $1 million paperback rights) and "Where Are the Children?" ($3,000 advance and $100,000 paperback rights).

"Did we ever drink champagne to celebrate the $100,000 paperback royalty on the first," she recalls. "Now I didn't have to worry about the kids' school bills."

Actually, "Where Are the Children?" was her second book. The first was "Aspire to the Heavens," a biography of George Washingtonwith a printing of 1,700 copies.

"Its a collectors item," she says with the smile of an author who now writes for millions of readers. "Most went to my relatives. The bookstores thought it was a prayer book and put it in the religion section."

The biogaphy grew out of a radio script that Clark wrote. As a widow with five children ranging in age from 5 to 13, she had to earn a living and began writing four-minute scripts for "Portrait of a Patriot," a five-a-week radio show with host bud Collier ("There was a teaser quiz at the beginning: Who installed the first bathtub in the White House? Who was a failure at 63 and later a world leader?")

She also turned out radio scripts fof Bess Meyerson's food show and a Hugh Downs driving-home show.

"But with radio, it was on and then gone I had written short stories. But I wondered if I could sustain a novel. The biography of George Washington showed me that I could.

So she began writing "Where Are the Children?" which was a best seller as a first novel in 1975. How did she find time for the lonely solitary pursuit of writing while caring for her five children?

"You don't put off writing," she explains. "I wrote one short story that I sold to Redbook while waiting in the dentist's office for the orthodontist to do one of my children's teeth. Your mind can always be at work, spinning stories, working out plots, while you are sorting diapers or vacuuming."

While attending the trials and reading the newspaper accounts that provide terror for her plots, Clark says: "I always ask: 'What could happen?" I go the next step from probable to possible."

For "Where Are the Children?" the spark of the idea came from Alice Crimmins trial, although the novel does not parallel the actual case.

"I just thought of a mother of children who had been murdered and then is tried for their murder," she says, taking the next step to the possible.

"A Stranger is Watching" drew from a real-life case in which a 4-year-old was the only witness to the murder of his mother by a psychotic killer passing through the community.

Clark is a master storyteller who builds her taut suspense in a limited time frame. "Where Are the Children?" begins at 9 a.m. and ends at 7 p.m. the same day. Only three days elapse in "A Stranger Is Watching."

"I believe in containment," she explains."I like something I can control. It's like Oedipus Rex saying: It's a lousy day. My wife committed suicide, I'm blinded and the kingdom is going to pot.'"

It was the pictures of Baby Louise, the first test-tube baby, that gave Clark the idea for "The Cradle Will Fall."

"There was Baby Louise's smiling picture in the evening paper. Two clinics now are opened. And I read inthe Ladies Home Journal that some doctors felt it was only time before there could be a human embryo implantation."

So Clark thought: "What could happen?" Her latest novel is a chilling tale of an obsessed gynecologist-obstetrician who uses unwitting patients to do research on the implantation of an aborted fetus to a host mother. It is a chilling tale that revolves around Katie DeMaio, a young prosecuting attorney recently widowed. While being treated for injuries from an auto accident, she wakes up, sedated, to what she first believes a nightmare -- a man putting a wrapped body in the trunk of a car.

"I always have expert advice. My daughter, Marilyn, who is a prosecuting attorney, checked the legal points," says Clark.

"I was on the phone for four hours with a doctor in Minnesota. I had written about having a transfusion ready. He said a doctor would not say that to another doctor in an emergency. It would be: "Have a bottle of o negative ready'"

While writing her three thrillers to best-selling succcess, Clark went back to college. Last year she earned a bachelor of arts degree in philosophy summa cum laude from Fordham university's Lincoln Center branch. Two years ago she married Raymond Ploetz, a divorced lawyer with four children of his own.

She divides her time between their farm, 30 miles outside minneapolis; her old suburban home in New Jersey, and a pied-a-terre across the street from Central Park. When she is writing, she gets up at 5 a.m. and writes for five hours at least three times a week.

She is well into her fourth suspense novel, "And Through the Wood." It is set on a farm in Minnesota.

"It has the scariest villian of all. I just love him," she says, and smiles.