Mannheim was born in Friedrichshain, a district of Berlin, where she studied drama and quickly became a popular figure appearing on stage in plays and musicals.

She appeared in several films there, including her role as the doomed spy Annabella Smith in Alfred Hitchcock's version of The 39 Steps (1935).

During the Second World War, she appeared in several films, as well as broadcasting propaganda to Germany with the Londoner Rundfunk [de] – including performing an anti-Hitler version of Lili Marleen, in 1943.

In 1955 she joined the cast of the British television series The Adventures of the Scarlet Pimpernel as Countess La Valliere, which starred her husband.

Mannheim was awarded the West German Commander's Cross of the Order of Merit in 1959, and made a Staatsschauspieler ("State Actor") for Berlin in 1963.