Born Irina Dziemiach, of Russian ancestry, in Pommeuse, Seine-et-Marne, she went to Paris and became a model. She made an appearance in a French film Julie la rousse (1959) and met Producer Darryl F. Zanuck, whose lover she became: he then cast her in his epic production, The Longest Day as a French resistance fighter. Her career continued with roles in OSS se déchaîne (1963), The Visit (1964), alongside Ingrid Bergman and Anthony Quinn, Un monsieur de compagnie (1964) with Catherine Deneuve and Jean-Pierre Cassel and Up from the Beach (1965) opposite Cliff Robertson and Red Buttons. In 1965, she played in La Métamorphose des cloportes, and seven roles in Those Magnificent Men in their Flying Machines, each one of a different nationality.
In 1964, she married Philippe Wahl, a Swiss Entrepreneur. Together they lived in Rome and Paris. After her divorce in 1979, she moved to the U.S. She died in Indianapolis, Indiana.
After making a few more films including Prudence and the Pill (1968) and Le Clan des Siciliens (The Sicilian Clan, 1969), with Jean Gabin and Alain Delon, and two Italian horror films in 1972, Demick's career faded and came to a standstill.
