Rudolf Gernreich was the only child of Siegmund Gernreich and Elisabeth (née Müller) Gernreich, a Jewish couple who lived in Vienna, Austria. His father was a stocking manufacturer who had served in World War I and who committed suicide when Rudi was eight years old.
After the German Anschluss (when Nazi Germany annexed Austria) on 12 March 1938, Hitler, among many other acts, banned nudity. Austrian citizens were advocates of exercising nude, a rejection of the over-civilized world. His mother took 16-year-old Rudi and escaped to the United States as Jewish refugees, settling in Los Angeles, California.
To survive, his mother baked pastries that Rudi sold door-to-door. His first job was washing bodies to prepare them for autopsy in the morgue of Cedars of Lebanon Hospital. He attended Los Angeles City College, where he studied art and apprenticed for a Seventh Avenue clothing manufacturer. He attended Los Angeles City College from 1938 to 1941, and at the Los Angeles Art Center School from 1941 to 1942.
A poor student, Gernreich fell in love with modern dance while working as an usher at a Martha Graham performance. He joined the Lester Horton company in 1942.
Gernreich's first real design success came with bold Pop Art-related fabric patterns and color combinations he created for Hoffman California Fabrics. He is the inventor of the women's topless bathing suit (monokini) in 1964.
In his later life, Gernreich devoted himself to gourmet soups. He is credited with a recipe for red-pepper soup, a cold soup served in red-pepper cases and garnished with caviar and lemon.
He is a co-founder of the Mattachine Society. Rudi kept his relationships private as he believed public acknowledgment of his homosexuality would negatively affect his fashion business.His known lovers were Harry Hay (1950–1952) and Oreste Pucciani (1953–1985; Gernreich's death). Rudi was diagnosed with lung cancer in January 1985 and died three months later, at age 62.