In the 1954 American thriller Night People, she appeared in a supporting role alongside Gregory Peck.

Koch also had major roles in the Hollywood films Four Girls in Town and Interlude, both released in 1957.

She remains perhaps best known internationally for Sergio Leone's 1964 production A Fistful of Dollars, which showcased her with Clint Eastwood as a civilian tormented by ruthless local gangsters, torn between her husband and child and the villains.

In Germany, she was probably best known for her many years of participation as one of the regular panelists in the highly popular TV game show Was bin ich?, the German adaption of the American TV show What's My Line?, which ran from the 1950s until 1988 and achieved ratings of up to 75% at its peak.

She also hosted other television shows, and in 2014, still had a medical advice program on radio.