Greer Garson
— Marie Curie
Walter Pidgeon
— Pierre Curie
Henry Travers
— Eugene Curie
Albert Bassermann
— Professor Jean Perot
Robert Walker
— David Le Gros
C. Aubrey Smith
— Lord Kelvin
May Whitty
— Madame Eugene Curie
Victor Francen
— President of University
Elsa Bassermann
— Madame Perot
Reginald Owen
— Dr. Becquerel
Van Johnson
— Reporter
Margaret O'Brien
— Irene Curie - Age 5
James Hilton
— Narration Spoken By (voice)
Mariska Aldrich
— Tall Woman (uncredited)
Tony Carson
— Man at Accident (uncredited)
Ruth Cherrington
— Swedish Queen (uncredited)
Ray Collins
— Lecturer (voice) (uncredited)
Guy D'Ennery
— Professor (uncredited)
George Davis
— Cart Driver (uncredited)
Harold De Becker
— Professor (uncredited)
Franz Dorfler
— Assistant Tailor (uncredited)
Justine Duney
— Woman at Accident (uncredited)
Tay Dunn
— Man at Accident (uncredited)
William Edmunds
— Cart Driver (uncredited)
Nestor Eristoff
— Board Member (uncredited)
Al Ferguson
— Man at Accident (uncredited)
Edward Fielding
— Board Member (uncredited)
Howard Freeman
— Prof. Constant (voice) (uncredited)
Linda Lee Gates
— Perot Granddaughter (uncredited)
Marie Louise Gates
— Perot Granddaughter (uncredited)