In 1969, Seymour appeared uncredited in her first film, Richard Attenborough's Oh! In 1970, Seymour appeared in her first major film role in the war drama The Only Way.
She played Lillian Stein, a Jewish woman seeking shelter from Nazi persecution.
Seymour returned to the big screen in the comedy Oh Heavenly Dog opposite Chevy Chase.
In 1980, Seymour played the role on stage of Constanze in Peter Shaffer's play Amadeus, opposite Ian Mc Kellen as Salieri and Tim Curry as Mozart.
The film was not released until its stop motion animation sequences had been completed in 1977.
In 1978, she appeared as Serina in the Battlestar Galactica film, and in the first five episodes of the television series.
In the 2000s, Seymour continued to work primarily in television.