Seymour was born Joyce Penelope Wilhelmina Frankenberg in Hayes, Middlesex, England, the daughter of Mieke, a nurse, and John Benjamin Frankenberg, an obstetrician. Her father was an English Jew whose family was from Poland, and her mother was a Dutch-born Protestant who was a prisoner of war during WWII. Seymour was educated at the independent The Arts Educational School in Tring, Hertfordshire, in England. She took on the stage name "Jane Seymour", also the name of King Henry VIII's third wife, at the age of 17.

Seymour has had a long acting career in both film and television, beginning in 1969 with an uncredited role in Richard Attenborough's film version of Oh! What a Lovely War. Soon afterward, she married Attenborough's son, Michael Attenborough. Her first major film role was as Lillian Stein, a Jewish woman seeking shelter from the Nazis, with a Danish family, in the 1970 war drama The Only Way.

From 1972 to 1973, she gained her first major TV role, as Emma Callon in the successful 1970s series The Onedin Line. During this time, she appeared as female lead Prima in the two-part TV mini-series Frankenstein: The True Story, and as Winston Churchill's lover Pamela Plowden, in another of the films, produced by her father-in-law, Young Winston. She also drew her first major international attention, as Bond girl Solitaire in the 1973 James Bond film, Live and Let Die. IGN ranked her as 10th in a Top 10 Bond Babes list.

Seymour divorced Michael Attenborough in 1973. She then took only two minor TV roles, until cast as Princess Farah in Sinbad and the Eye of the Tiger, the third part of Ray Harryhausen's Sinbad trilogy, in 1975. (The film was not released, however, until its stop motion animation sequences had been completed in 1977.) In 1978, she played Serina, in the Battlestar Galactica motion picture, and then, in the first two episodes of the series that followed, until the character was killed. In 1981, she was cast as Cathy Ames, in the TV miniseries of John Steinbeck's East of Eden. She also played the role of an undercover reporter in a TV movie about the Dallas Cowboys Cheerleaders.

In 1980, Seymour returned to the big screen in the comedy Oh Heavenly Dog opposite Chevy Chase, and as Elise McKenna in the romantic fantasy Somewhere in Time opposite Christopher Reeve. In 1982 she also starred in The Scarlet Pimpernel, co-starring Anthony Andrews and Ian McKellen. Seymour appeared nude in the 1984 film, Lassiter, co-starring Tom Selleck, but the film was a box office and commercial failure. In 1987, Seymour was the subject of a pictorial in Playboy magazine, although she did not actually pose nude.

Seymour won the female lead in the 12-part TV-miniseries, War and Remembrance (1988), in which she played Natalie Henry, an American Jewish woman trapped in Europe during World War II. The series was based on the successful novel by Herman Wouk, and is noted for its accurate, and graphic, depiction of the Holocaust.

In 1989, on the occasion of the 200th anniversary of the French Revolution, Seymour appeared in the television movie, La révolution française (filmed in both French and English). Seymour appeared as the doomed French queen, Marie Antoinette; the actress' two children, Katherine and Sean, appeared as the queen's children.

Seymour continued to take numerous roles in TV movies and series, most notably as Dr. Michaela "Mike" Quinn in the TV series, Dr. Quinn, Medicine Woman, and its TV-movie-sequels (1993—2001), through which she met her fourth husband, actor-director James Keach. In 2004, she made several guest appearances in the WB Network series, Smallville, playing Genevieve Teague, the wealthy, scheming, mother of Jason Teague (Jensen Ackles).

Seymour returned to the big screen in 2005, playing Kathleen Cleary, wife of fictional US Secretary of the Treasury William Cleary (Christopher Walken), in the comedy, Wedding Crashers. She returned to TV in the short-lived WB series Modern Men, broadcast in spring 2006.

In fall 2006, Seymour guest-starred as a law-school-professor on an episode of the CBS sitcom How I Met Your Mother, and as a wealthy client, on the Fox legal drama, Justice. In 2007, she guest-starred in the ABC sitcom, In Case of Emergency, which starred Lori Loughlin and Jonathan Silverman. She also appeared in ITV's Marple: Ordeal By Innocence, based on the Agatha Christie novel. She was a contestant on season five of the US reality show, Dancing with the Stars; she finished in sixth place, along with her partner, Tony Dovolani.

In 2008, she replaced Selina Scott as the new face of Country Casuals.

Seymour has heterochromia - her right eye is hazel, and her left is green. In 2007, she admitted to having undergone plastic surgery, including breast augmentation, and blepharoplasty.

Seymour has been married four times:

• 1971-1973: Michael Attenborough
• 1977-1978: Geoffrey Planer
• 1981-1992: David Flynn (with whom she had two children, Katherine Flynn, born January 7, 1982 and Sean, born 1986. Her daughter and stepdaughter, Jenni Flynn, appeared with her in the infomercial for cosmetics line, "Natural Advantage by Jane Seymour.")
• 1993 to present: James Keach (with whom she had twins, Johnny and Kris, born November 30, 1995, and named after family friends, Johnny Cash and Christopher Reeve)

Seymour dated musician Peter Cetera, former bass guitarist and lead singer of the rock band, Chicago during 1991. Her relationship with Cetera was publicized in a number of celebrity gossip magazines and supermarket tabloids of the time.

In 1984, Seymour bought, with then-husband, David Flynn, the Grade One listed St Catherine's Court for £350,000, located in the village of St Catherine, near Bath, Somerset. After spending £3 million on refurbishments, she spent her summers at the house, and her winters in Malibu. After her divorce from Flynn, and marriage to Keach, she spent more time in the US, and made little use of the house, so she began to rent it out. In 1996, during that season's filming for Dr Quinn, Medicine Woman, she rented it, to English rock group, Radiohead, who recorded their album, OK Computer, at the house. Another famous group that occupied the home are The Cure. In May 2007, she was granted a 24-hour alcohol and entertainment licence, under new UK regulations. However, this caused much disturbance with neighbours, who claimed the access lane was too narrow, and the noise too excessive. Seymour won the court battle, but sold the house in November 2007.

Seymour was named an Officer of the Order of the British Empire by Queen Elizabeth II, in 2000.

Seymour is a patron of Meningitis UK.

Seymour is a celebrity ambassador, for the non-profit organisation, Childhelp. She regularly makes appearances at fund raisers, and events for the child abuse prevention, and treatment organisation, and is an ardent supporter. In 2007, she sponsored a children's Art Pillow contest, as part of the Jane Seymour Collection. 100% of the proceeds went to Childhelp.

An allergic reaction to antibiotic medicine on a film shoot in Spain almost killed her, and the scrape with death profoundly changed her whole outlook on life. Seymour explains: "I saw the white light and I saw, from the corner of the room, them trying to resuscitate me, and I saw a syringe, with blood in it. It did change my whole life, because, when you die, I realised, you take nothing with you, except for what you've done."

On December 2, 2008, she was honoured by the Christopher and Dana Reeve Foundation, in a ceremony in Los Angeles, for her work with people with disabilities.

In late 2009, furniture supplier AICO and founder/CEO Michael Amini announced that they will collaborate with Jane Seymour to co-brand a new line of fine furniture. The line of furniture was unveiled in February 2010 and featured three collections consisting of a traditional collection, a casual European collection, and a Hollywood glamour collection.

She currently resides in Malibu, California, with her husband, and twin sons.