She took the stage name Jane Seymour at the age of 17. She was educated at the independent The Arts Educational School in Tring, Hertfordshire, in England. 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 Christian 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 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.

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 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.

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.

Seymour returned to the big screen in 2005 with playing Kathleen Cleary, wife of fictional U.S. 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.