Vos Savant was born Marilyn Mach in St. Louis, Missouri to Joseph Mach and Marina vos Savant, who had immigrated to the United States from Germany and Italy respectively. Vos Savant believes that both men and women should keep their premarital surnames for life, with sons taking their fathers' surnames and daughters their mothers'. The word "savant", meaning a person of learning, appears twice in her family: her maternal grandmother's maiden name was Savant, while her maternal grandfather's surname was vos Savant. Vos Savant is of German and Italian ancestry, and is a descendant of physicist and philosopher Ernst Mach.

As a teenager, vos Savant used to spend her time working in her father's general store and enjoyed writing and reading. She sometimes wrote articles and subsequently published them under a pseudonym in the local newspaper, stating that she did not want to misuse her name for work that she perceived to be imperfect. When she was sixteen years old, vos Savant married a university student, but the marriage ended in a divorce when she was in her twenties. Her second marriage ended when she was 35.

Vos Savant studied philosophy at the Washington University in St. Louis despite her parents' desire for a more useful subject. After two years, she dropped out to help with a family investment business, seeking financial freedom to pursue a career in writing.

Vos Savant moved to New York City in the 1980s. Before her weekly column in Parade, vos Savant wrote the Omni I.Q. Quiz Contest for Omni, which contains "I.Q. quizzes" and expositions on intelligence and intelligence testing.

Vos Savant lives in New York City with her husband Robert Jarvik, one of the developers of the Jarvik artificial heart. Vos Savant is Chief Financial Officer of Jarvik Heart, Inc., and is involved in cardiovascular disease research and prevention. She has served on the Board of Directors of the National Council on Economic Education and on the advisory boards of the National Association for Gifted Children and the National Women's History Museum, which in 1998 gave her a "Women Making History" Award, citing "her contribution to changing stereotypes about women." She was named by Toastmasters International as one of the "Five Outstanding Speakers of 1999," and in 2003 received an honorary Doctor of Letters from The College of New Jersey.

In 1985, Guinness Book of World Records accepted vos Savant's IQ score of 228 and gave her the record for "Highest IQ". She was listed in that category from 1986 to 1989. She was inducted into the Guinness Book of World Records Hall of Fame in 1988. Guinness retired the category of "Highest IQ" in 1990, after concluding that IQ tests are not reliable enough to designate a single world record holder. The listing gave her nationwide attention and instigated her rise to fame.

Guinness cites vos Savant's performance on two intelligence tests, the Stanford-Binet and the Mega Test. She was administered the 1937 Stanford-Binet, Second Revision test at age ten, which obtained ratio IQ scores (by dividing the subject's mental age as assessed by the test by chronological age, then multiplying the quotient by 100). Vos Savant says her first test was in September 1956, and measured her ceiling mental age at 22 years and 10 months (22-10+), yielding an IQ of 228. The IQ calculation of 228 was listed in Guinness Book of World Records, listed in the short biographies in her books, and is the one she gives in interviews. Sometimes, a rounded value of 230 appears.

Ronald K. Hoeflin calculated her IQ at 218 by using 10-6+ for chronological age and 22-11+ for mental age. The Second Revision Stanford-Binet ceiling was 22 years and 10 months, not 11 months. A 10 years and 6 months chronological age corresponds to neither the age in accounts by vos Savant nor the school records cited by Baumgold. She has commented on reports mentioning varying IQ scores she was said to have obtained..

Alan S. Kaufman, an author of IQ tests and of books about IQ testing, writes in IQ Testing 101 that "Miss Savant was given an old version of the Stanford-Binet (Terman & Merrill 1937), which did, indeed, use the antiquated formula of MA/CA × 100. But in the test manual's norms, the Binet does not permit IQs to rise above 170 at any age, child or adult. And the authors of the old Binet stated: 'Beyond fifteen the mental ages are entirely artificial and are to be thought of as simply numerical scores.' (Terman & Merrill 1937). . . . the psychologist who came up with an IQ of 228 committed an extrapolation of a misconception, thereby violating almost every rule imaginable concerning the meaning of IQs."

The second test reported by Guinness is the Mega Test, designed by Ronald K. Hoeflin, administered to vos Savant in the mid-1980s as an adult. The Mega Test yields deviation IQ values obtained by multiplying the subjects normalized z-score, or the rarity of the raw test score, by a constant standard deviation, and adding the product to 100, with vos Savant's raw score reported by Hoeflin to be 46 out of a possible 48, with 5.4 z-score, and standard deviation of 16, arriving at a 186 IQ in the 99.999997 percentile, with a rarity of 1 in 30 million. The Mega Test has been criticized by professional psychologists as improperly designed and scored, "nothing short of number pulverization."

Although vos Savant's IQ scores are among the highest recorded, the more extravagant sources, stating that she is the smartest person in the world and was a child prodigy, have been received with skepticism. Vos Savant herself says she values IQ tests as measurements of a variety of mental abilities and believes intelligence itself involves so many factors that "attempts to measure it are useless."

Vos Savant has held memberships with the high-IQ societies, Mensa International and the Prometheus Society.

Vos Savant is most widely known for her weekly column in Parade, "Ask Marilyn". Vos Savant's listing in the 1986 Guinness Book of World Records brought her widespread media attention. Parade ran a profile of vos Savant with a selection of questions from Parade readers and her answers. Parade continued to receive questions, so "Ask Marilyn" was made into a weekly column.

In "Ask Marilyn", vos Savant answers questions from readers on a wide range of chiefly academic subjects, solves mathematical or logical or vocabulary puzzles posed by readers, occasionally answers requests for advice with logic, and includes quizzes and puzzles devised by vos Savant. Aside from the weekly printed column, "Ask Marilyn" is a daily online column which supplements the printed column by resolving controversial answers, correcting mistakes, expanding answers, reposting previous answers, and answering additional questions.

Three of her books (Ask Marilyn, More Marilyn, and Of Course, I'm for Monogamy) are compilations of questions and answers from "Ask Marilyn"; and The Power of Logical Thinking includes many questions and answers from the column.

A few months after the announcement by Andrew Wiles that he had proved Fermat's Last Theorem, vos Savant published her book The World's Most Famous Math Problem in October 1993. The book surveys the history of Fermat's last theorem as well as other mathematical mysteries. Controversy came from the book's criticism of Wiles' proof; vos Savant was accused of misunderstanding mathematical induction, proof by contradiction, and imaginary numbers.

Her assertion that Wiles' proof should be rejected for its use of non-Euclidean geometry was especially contested. Specifically, she argued that because "the chain of proof is based in hyperbolic geometry," and because squaring the circle is considered a "famous impossibility" despite being possible in hyperbolic geometry, then "if we reject a hyperbolic method of squaring the circle, we should also reject a hyperbolic proof of Fermat's last theorem."

Mathematicians pointed to differences between the two cases, distinguishing the use of hyperbolic geometry as a tool for proving Fermat's last theorem and from its use as a setting for squaring the circle: squaring the circle in hyperbolic geometry is a different problem from that of squaring it in Euclidean geometry. She was criticized for rejecting hyperbolic geometry as a satisfactory basis for Wiles' proof, with critics pointing out that axiomatic set theory (rather than Euclidean geometry) is now the accepted foundation of mathematical proofs and that set theory is sufficiently robust to encompass both Euclidean and non-Euclidean geometry as well as geometry and adding numbers.

In a July 1995 addendum to the book, vos Savant retracts the argument, writing that she had viewed the theorem as "an intellectual challenge—'to find a proof with Fermat's tools.'" Fermat claimed to have a proof he couldn't fit in the margins where he wrote his theorem. If he really had a proof, it would presumably be Euclidean. Therefore, Wiles may have proven the theorem but Fermat's proof remains undiscovered, if it ever really existed. She is now willing to agree that there are no restrictions on what tools may be used.

The Monty Hall problem

Perhaps the best-known event involving vos Savant began with a question in her 9 September 1990 column:

Suppose you're on a game show, and you're given the choice of three doors. Behind one door is a car, behind the others, goats. You pick a door, say #1, and the host, who knows what's behind the doors, opens another door, say #3, which has a goat. He says to you, "Do you want to pick door #2?" Is it to your advantage to switch your choice of doors?

—Craig F. Whitaker Columbia, Maryland

This question, named "the Monty Hall problem" because of its similarity to scenarios on the game show Let's Make a Deal, existed long before being posed to vos Savant, but was brought to nationwide attention by her column. Vos Savant answered arguing that the selection should be switched to door #2 because it has a 2/3 chance of success, while door #1 has just 1/3. Or to summarise, 2/3 of the time the opened door #3 will indicate the location of door with the car (the door you hadn't picked and the one not opened by the host). Only 1/3 of the time will the opened door #3 mislead you into changing from the winning door to a losing door. These probabilities assume you change your choice each time door #3 is opened, and that the host always opens a door with a goat. This response provoked letters of thousands of readers, nearly all arguing doors #1 and #2 each have an equal chance of success. A follow-up column reaffirming her position served only to intensify the debate and soon became a feature article on the front page of The New York Times. Among the ranks of dissenting arguments were hundreds of academics and mathematicians.

Under the most common interpretation of the problem where the host opens a losing door and offers a switch, vos Savant's answer is correct because her interpretation assumes the host will always avoid the door with the prize. However, having the host opening a door at random, or offering a switch only if the initial choice is correct, is a completely different problem, and is not the question for which she provided a solution. Vos Savant addressed these issues by writing the following in Parade Magazine, "...the original answer defines certain conditions, the most significant of which is that the host always opens a losing door on purpose. Anything else is a different question." In vos Savant's second followup, she went further into an explanation of her assumptions and reasoning, and called on school teachers to present the problem to each of their classrooms. In her final column on the problem, she announced the results of the more than a thousand school experiments. Nearly 100% of the results concluded that it pays to switch. Of the readers who wrote computer simulations of the problem, 97% reached the same conclusion. A majority of respondents now agree with her original solution, with half of the published letters declaring the letter writers had changed their minds.

This problem has been used in many different books, movies, etc. including the movie 21 and the novel The Curious Incident of the Dog in the Night-time.

"Two boys" problem

Like the Monty Hall problem, the "two boys" or "second-sibling" problem predates Ask Marilyn, but generated controversy in the column, first appearing there in 1991-92 in the context of baby beagles:

A shopkeeper says she has two new baby beagles to show you, but she doesn't know whether they're male, female, or a pair. You tell her that you want only a male, and she telephones the fellow who's giving them a bath. "Is at least one a male?" she asks him. "Yes!" she informs you with a smile. What is the probability that the other one is a male?—Stephen I. Geller, Pasadena, California

When vos Savant replied "One out of three", readers wrote to argue that the odds were fifty-fifty. In a follow-up, she defended her answer, observing that "If we could shake a pair of puppies out of a cup the way we do dice, there are four ways they could land", in three of which at least one is male, but in only one of which both are male. See Boy or Girl paradox for solution details.

The problem re-emerged in 1996-97 with two cases juxtaposed:

Say that a woman and a man (who are unrelated) each has two children. We know that at least one of the woman's children is a boy and that the man's oldest child is a boy. Can you explain why the chances that the woman has two boys do not equal the chances that the man has two boys? My algebra teacher insists that the probability is greater that the man has two boys, but I think the chances may be the same. What do you think?

Vos Savant agreed with the algebra teacher, writing that the chances are only 1 out of 3 that the woman has two boys, but 1 out of 2 that the man has two boys. Readers argued for 1 out of 2 in both cases, prompting multiple follow-ups. Finally, vos Savant started a survey, calling on women readers (with exactly two children and at least one boy) and male readers (with exactly two children - the elder a boy) to tell her the sex of both children. With almost eighteen thousand responses, the results showed 35.9% of them having two boys.

• 1985 — Omni I.Q. Quiz Contest
• 1990 — Brain Building: Exercising Yourself Smarter (co-written with Leonore Fleischer)
• 1992 — Ask Marilyn: Answers to America's Most Frequently Asked Questions
• 1993 — The World's Most Famous Math Problem: The Proof of Fermat's Last Theorem and Other Mathematical Mysteries
• 1994 — More Marilyn: Some Like It Bright!
• 1994 — "I've Forgotten Everything I Learned in School!": A Refresher Course to Help You Reclaim Your Education
• 1996 — Of Course I'm for Monogamy: I'm Also for Everlasting Peace and an End to Taxes
• 1996 — The Power of Logical Thinking: Easy Lessons in the Art of Reasoningand Hard Facts about Its Absence in Our Lives
• 2000 — The Art of Spelling: The Madness and the Method
• 2002 — Growing Up: A Classic American Childhood

In addition to her published works, vos Savant has written a collection of humorous short stories called Short Shorts, a stage play called It Was Poppa's Will, and two novels: a satire of a dozen classical civilizations in history called The Re-Creation, and a futuristic political fantasy, as yet untitled.